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Diffstat (limited to 'src/liblzma/simple/riscv.c')
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diff --git a/src/liblzma/simple/riscv.c b/src/liblzma/simple/riscv.c new file mode 100644 index 0000000..b18df8b --- /dev/null +++ b/src/liblzma/simple/riscv.c @@ -0,0 +1,755 @@ +// SPDX-License-Identifier: 0BSD + +/////////////////////////////////////////////////////////////////////////////// +// +/// \file riscv.c +/// \brief Filter for 32-bit/64-bit little/big endian RISC-V binaries +/// +/// This converts program counter relative addresses in function calls +/// (JAL, AUIPC+JALR), address calculation of functions and global +/// variables (AUIPC+ADDI), loads (AUIPC+load), and stores (AUIPC+store). +/// +/// For AUIPC+inst2 pairs, the paired instruction checking is fairly relaxed. +/// The paired instruction opcode must only have its lowest two bits set, +/// meaning it will convert any paired instruction that is not a 16-bit +/// compressed instruction. This was shown to be enough to keep the number +/// of false matches low while improving code size and speed. +// +// Authors: Lasse Collin +// Jia Tan +// +// Special thanks: +// +// - Chien Wong <m@xv97.com> provided a few early versions of RISC-V +// filter variants along with test files and benchmark results. +// +// - Igor Pavlov helped a lot in the filter design, getting it both +// faster and smaller. The implementation here is still independently +// written, not based on LZMA SDK. +// +/////////////////////////////////////////////////////////////////////////////// + +/* + +RISC-V filtering +================ + + RV32I and RV64I, possibly combined with extensions C, Zfh, F, D, + and Q, are identical enough that the same filter works for both. + + The instruction encoding is always little endian, even on systems + with big endian data access. Thus the same filter works for both + endiannesses. + + The following instructions have program counter relative + (pc-relative) behavior: + +JAL +--- + + JAL is used for function calls (including tail calls) and + unconditional jumps within functions. Jumps within functions + aren't useful to filter because the absolute addresses often + appear only once or at most a few times. Tail calls and jumps + within functions look the same to a simple filter so neither + are filtered, that is, JAL x0 is ignored (the ABI name of the + register x0 is "zero"). + + Almost all calls store the return address to register x1 (ra) + or x5 (t0). To reduce false matches when the filter is applied + to non-code data, only the JAL instructions that use x1 or x5 + are converted. JAL has pc-relative range of +/-1 MiB so longer + calls and jumps need another method (AUIPC+JALR). + +C.J and C.JAL +------------- + + C.J and C.JAL have pc-relative range of +/-2 KiB. + + C.J is for tail calls and jumps within functions and isn't + filtered for the reasons mentioned for JAL x0. + + C.JAL is an RV32C-only instruction. Its encoding overlaps with + RV64C-only C.ADDIW which is a common instruction. So if filtering + C.JAL was useful (it wasn't tested) then a separate filter would + be needed for RV32 and RV64. Also, false positives would be a + significant problem when the filter is applied to non-code data + because C.JAL needs only five bits to match. Thus, this filter + doesn't modify C.JAL instructions. + +BEQ, BNE, BLT, BGE, BLTU, BGEU, C.BEQZ, and C.BNEZ +-------------------------------------------------- + + These are conditional branches with pc-relative range + of +/-4 KiB (+/-256 B for C.*). The absolute addresses often + appear only once and very short distances are the most common, + so filtering these instructions would make compression worse. + +AUIPC with rd != x0 +------------------- + + AUIPC is paired with a second instruction (inst2) to do + pc-relative jumps, calls, loads, stores, and for taking + an address of a symbol. AUIPC has a 20-bit immediate and + the possible inst2 choices have a 12-bit immediate. + + AUIPC stores pc + 20-bit signed immediate to a register. + The immediate encodes a multiple of 4 KiB so AUIPC itself + has a pc-relative range of +/-2 GiB. AUIPC does *NOT* set + the lowest 12 bits of the result to zero! This means that + the 12-bit immediate in inst2 cannot just include the lowest + 12 bits of the absolute address as is; the immediate has to + compensate for the lowest 12 bits that AUIPC copies from the + program counter. This means that a good filter has to convert + not only AUIPC but also the paired inst2. + + A strict filter would focus on filtering the following + AUIPC+inst2 pairs: + + - AUIPC+JALR: Function calls, including tail calls. + + - AUIPC+ADDI: Calculating the address of a function + or a global variable. + + - AUIPC+load/store from the base instruction sets + (RV32I, RV64I) or from the floating point extensions + Zfh, F, D, and Q: + * RV32I: LB, LH, LW, LBU, LHU, SB, SH, SW + * RV64I has also: LD, LWU, SD + * Zfh: FLH, FSH + * F: FLW, FSW + * D: FLD, FSD + * Q: FLQ, FSQ + + NOTE: AUIPC+inst2 can only be a pair if AUIPC's rd specifies + the same register as inst2's rs1. + + Instead of strictly accepting only the above instructions as inst2, + this filter uses a much simpler condition: the lowest two bits of + inst2 must be set, that is, inst2 must not be a 16-bit compressed + instruction. So this will accept all 32-bit and possible future + extended instructions as a pair to AUIPC if the bits in AUIPC's + rd [11:7] match the bits [19:15] in inst2 (the bits that I-type and + S-type instructions use for rs1). Testing showed that this relaxed + condition for inst2 did not consistently or significantly affect + compression ratio but it reduced code size and improved speed. + + Additionally, the paired instruction is always treated as an I-type + instruction. The S-type instructions used by stores (SB, SH, SW, + etc.) place the lowest 5 bits of the immediate in a different + location than I-type instructions. AUIPC+store pairs are less + common than other pairs, and testing showed that the extra + code required to handle S-type instructions was not worth the + compression ratio gained. + + AUIPC+inst2 don't necessarily appear sequentially next to each + other although very often they do. Especially AUIPC+JALR are + sequential as that may allow instruction fusion in processors + (and perhaps help branch prediction as a fused AUIPC+JALR is + a direct branch while JALR alone is an indirect branch). + + Clang 16 can generate code where AUIPC+inst2 is split: + + - AUIPC is outside a loop and inst2 (load/store) is inside + the loop. This way the AUIPC instruction needs to be + executed only once. + + - Load-modify-store may have AUIPC for the load and the same + AUIPC-result is used for the store too. This may get combined + with AUIPC being outside the loop. + + - AUIPC is before a conditional branch and inst2 is hundreds + of bytes away at the branch target. + + - Inner and outer pair: + + auipc a1,0x2f + auipc a2,0x3d + ld a2,-500(a2) + addi a1,a1,-233 + + - Many split pairs with an untaken conditional branch between: + + auipc s9,0x1613 # Pair 1 + auipc s4,0x1613 # Pair 2 + auipc s6,0x1613 # Pair 3 + auipc s10,0x1613 # Pair 4 + beqz a5,a3baae + ld a0,0(a6) + ld a6,246(s9) # Pair 1 + ld a1,250(s4) # Pair 2 + ld a3,254(s6) # Pair 3 + ld a4,258(s10) # Pair 4 + + It's not possible to find all split pairs in a filter like this. + At least in 2024, simple sequential pairs are 99 % of AUIPC uses + so filtering only such pairs gives good results and makes the + filter simpler. However, it's possible that future compilers will + produce different code where sequential pairs aren't as common. + + This filter doesn't convert AUIPC instructions alone because: + + (1) The conversion would be off-by-one (or off-by-4096) half the + time because the lowest 12 bits from inst2 (inst2_imm12) + aren't known. We only know that the absolute address is + pc + AUIPC_imm20 + [-2048, +2047] but there is no way to + know the exact 4096-byte multiple (or 4096 * n + 2048): + there are always two possibilities because AUIPC copies + the 12 lowest bits from pc instead of zeroing them. + + NOTE: The sign-extension of inst2_imm12 adds a tiny bit + of extra complexity to AUIPC math in general but it's not + the reason for this problem. The sign-extension only changes + the relative position of the pc-relative 4096-byte window. + + (2) Matching AUIPC instruction alone requires only seven bits. + When the filter is applied to non-code data, that leads + to many false positives which make compression worse. + As long as most AUIPC+inst2 pairs appear as two consecutive + instructions, converting only such pairs gives better results. + + In assembly, AUIPC+inst2 tend to look like this: + + # Call: + auipc ra, 0x12345 + jalr ra, -42(ra) + + # Tail call: + auipc t1, 0x12345 + jalr zero, -42(t1) + + # Getting the absolute address: + auipc a0, 0x12345 + addi a0, a0, -42 + + # rd of inst2 isn't necessarily the same as rs1 even + # in cases where there is no reason to preserve rs1. + auipc a0, 0x12345 + addi a1, a0, -42 + + As of 2024, 16-bit instructions from the C extension don't + appear as inst2. The RISC-V psABI doesn't list AUIPC+C.* as + a linker relaxation type explicitly but it's not disallowed + either. Usefulness is limited as most of the time the lowest + 12 bits won't fit in a C instruction. This filter doesn't + support AUIPC+C.* combinations because this makes the filter + simpler, there are no test files, and it hopefully will never + be needed anyway. + + (Compare AUIPC to ARM64 where ADRP does set the lowest 12 bits + to zero. The paired instruction has the lowest 12 bits of the + absolute address as is in a zero-extended immediate. Thus the + ARM64 filter doesn't need to care about the instructions that + are paired with ADRP. An off-by-4096 issue can still occur if + the code section isn't aligned with the filter's start offset. + It's not a problem with standalone ELF files but Windows PE + files need start_offset=3072 for best results. Also, a .tar + stores files with 512-byte alignment so most of the time it + won't be the best for ARM64.) + +AUIPC with rd == x0 +------------------- + + AUIPC instructions with rd=x0 are reserved for HINTs in the base + instruction set. Such AUIPC instructions are never filtered. + + As of January 2024, it seems likely that AUIPC with rd=x0 will + be used for landing pads (pseudoinstruction LPAD). LPAD is used + to mark valid targets for indirect jumps (for JALR), for example, + beginnings of functions. The 20-bit immediate in LPAD instruction + is a label, not a pc-relative address. Thus it would be + counterproductive to convert AUIPC instructions with rd=x0. + + Often the next instruction after LPAD won't have rs1=x0 and thus + the filtering would be skipped for that reason alone. However, + it's not good to rely on this. For example, consider a function + that begins like this: + + int foo(int i) + { + if (i <= 234) { + ... + } + + A compiler may generate something like this: + + lpad 0x54321 + li a5, 234 + bgt a0, a5, .L2 + + Converting the pseudoinstructions to raw instructions: + + auipc x0, 0x54321 + addi x15, x0, 234 + blt x15, x10, .L2 + + In this case the filter would undesirably convert the AUIPC+ADDI + pair if the filter didn't explicitly skip AUIPC instructions + that have rd=x0. + +*/ + + +#include "simple_private.h" + + +// This checks two conditions at once: +// - AUIPC rd == inst2 rs1. +// - inst2 opcode has the lowest two bits set. +// +// The 8 bit left shift aligns the rd of AUIPC with the rs1 of inst2. +// By XORing the registers, any non-zero value in those bits indicates the +// registers are not equal and thus not an AUIPC pair. Subtracting 3 from +// inst2 will zero out the first two opcode bits only when they are set. +// The mask tests if any of the register or opcode bits are set (and thus +// not an AUIPC pair). +// +// Alternative expression: (((((auipc) << 8) ^ (inst2)) & 0xF8003) != 3) +#define NOT_AUIPC_PAIR(auipc, inst2) \ + ((((auipc) << 8) ^ ((inst2) - 3)) & 0xF8003) + +// This macro checks multiple conditions: +// (1) AUIPC rd [11:7] == x2 (special rd value). +// (2) AUIPC bits 12 and 13 set (the lowest two opcode bits of packed inst2). +// (3) inst2_rs1 doesn't equal x0 or x2 because the opposite +// conversion is only done when +// auipc_rd != x0 && +// auipc_rd != x2 && +// auipc_rd == inst2_rs1. +// +// The left-hand side takes care of (1) and (2). +// (a) The lowest 7 bits are already known to be AUIPC so subtracting 0x17 +// makes those bits zeros. +// (b) If AUIPC rd equals x2, subtracting 0x100 makes bits [11:7] zeros. +// If rd doesn't equal x2, then there will be at least one non-zero bit +// and the next step (c) is irrelevant. +// (c) If the lowest two opcode bits of the packed inst2 are set in [13:12], +// then subtracting 0x3000 will make those bits zeros. Otherwise there +// will be at least one non-zero bit. +// +// The shift by 18 removes the high bits from the final '>=' comparison and +// ensures that any non-zero result will be larger than any possible result +// from the right-hand side of the comparison. The cast ensures that the +// left-hand side didn't get promoted to a larger type than uint32_t. +// +// On the right-hand side, inst2_rs1 & 0x1D will be non-zero as long as +// inst2_rs1 is not x0 or x2. +// +// The final '>=' comparison will make the expression true if: +// - The subtraction caused any bits to be set (special AUIPC rd value not +// used or inst2 opcode bits not set). (non-zero >= non-zero or 0) +// - The subtraction did not cause any bits to be set but inst2_rs1 was +// x0 or x2. (0 >= 0) +#define NOT_SPECIAL_AUIPC(auipc, inst2_rs1) \ + ((uint32_t)(((auipc) - 0x3117) << 18) >= ((inst2_rs1) & 0x1D)) + + +// The encode and decode functions are split for this filter because of the +// AUIPC+inst2 filtering. This filter design allows a decoder-only +// implementation to be smaller than alternative designs. + +#ifdef HAVE_ENCODER_RISCV +static size_t +riscv_encode(void *simple lzma_attribute((__unused__)), + uint32_t now_pos, + bool is_encoder lzma_attribute((__unused__)), + uint8_t *buffer, size_t size) +{ + // Avoid using i + 8 <= size in the loop condition. + // + // NOTE: If there is a JAL in the last six bytes of the stream, it + // won't be converted. This is intentional to keep the code simpler. + if (size < 8) + return 0; + + size -= 8; + + size_t i; + + // The loop is advanced by 2 bytes every iteration since the + // instruction stream may include 16-bit instructions (C extension). + for (i = 0; i <= size; i += 2) { + uint32_t inst = buffer[i]; + + if (inst == 0xEF) { + // JAL + const uint32_t b1 = buffer[i + 1]; + + // Only filter rd=x1(ra) and rd=x5(t0). + if ((b1 & 0x0D) != 0) + continue; + + // The 20-bit immediate is in four pieces. + // The encoder stores it in big endian form + // since it improves compression slightly. + const uint32_t b2 = buffer[i + 2]; + const uint32_t b3 = buffer[i + 3]; + const uint32_t pc = now_pos + (uint32_t)i; + +// The following chart shows the highest three bytes of JAL, focusing on +// the 20-bit immediate field [31:12]. The first row of numbers is the +// bit position in a 32-bit little endian instruction. The second row of +// numbers shows the order of the immediate field in a J-type instruction. +// The last row is the bit number in each byte. +// +// To determine the amount to shift each bit, subtract the value in +// the last row from the value in the second last row. If the number +// is positive, shift left. If negative, shift right. +// +// For example, at the rightmost side of the chart, the bit 4 in b1 is +// the bit 12 of the address. Thus that bit needs to be shifted left +// by 12 - 4 = 8 bits to put it in the right place in the addr variable. +// +// NOTE: The immediate of a J-type instruction holds bits [20:1] of +// the address. The bit [0] is always 0 and not part of the immediate. +// +// | b3 | b2 | b1 | +// | 31 30 29 28 27 26 25 24 | 23 22 21 20 19 18 17 16 | 15 14 13 12 x x x x | +// | 20 10 9 8 7 6 5 4 | 3 2 1 11 19 18 17 16 | 15 14 13 12 x x x x | +// | 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 | 7 6 5 4 x x x x | + + uint32_t addr = ((b1 & 0xF0) << 8) + | ((b2 & 0x0F) << 16) + | ((b2 & 0x10) << 7) + | ((b2 & 0xE0) >> 4) + | ((b3 & 0x7F) << 4) + | ((b3 & 0x80) << 13); + + addr += pc; + + buffer[i + 1] = (uint8_t)((b1 & 0x0F) + | ((addr >> 13) & 0xF0)); + + buffer[i + 2] = (uint8_t)(addr >> 9); + buffer[i + 3] = (uint8_t)(addr >> 1); + + // The "-2" is included because the for-loop will + // always increment by 2. In this case, we want to + // skip an extra 2 bytes since we used 4 bytes + // of input. + i += 4 - 2; + + } else if ((inst & 0x7F) == 0x17) { + // AUIPC + inst |= (uint32_t)buffer[i + 1] << 8; + inst |= (uint32_t)buffer[i + 2] << 16; + inst |= (uint32_t)buffer[i + 3] << 24; + + // Branch based on AUIPC's rd. The bitmask test does + // the same thing as this: + // + // const uint32_t auipc_rd = (inst >> 7) & 0x1F; + // if (auipc_rd != 0 && auipc_rd != 2) { + if (inst & 0xE80) { + // AUIPC's rd doesn't equal x0 or x2. + + // Check if AUIPC+inst2 are a pair. + uint32_t inst2 = read32le(buffer + i + 4); + + if (NOT_AUIPC_PAIR(inst, inst2)) { + // The NOT_AUIPC_PAIR macro allows + // a false AUIPC+AUIPC pair if the + // bits [19:15] (where rs1 would be) + // in the second AUIPC match the rd + // of the first AUIPC. + // + // We must skip enough forward so + // that the first two bytes of the + // second AUIPC cannot get converted. + // Such a conversion could make the + // current pair become a valid pair + // which would desync the decoder. + // + // Skipping six bytes is enough even + // though the above condition looks + // at the lowest four bits of the + // buffer[i + 6] too. This is safe + // because this filter never changes + // those bits if a conversion at + // that position is done. + i += 6 - 2; + continue; + } + + // Convert AUIPC+inst2 to a special format: + // + // - The lowest 7 bits [6:0] retain the + // AUIPC opcode. + // + // - The rd [11:7] is set to x2(sp). x2 is + // used as the stack pointer so AUIPC with + // rd=x2 should be very rare in real-world + // executables. + // + // - The remaining 20 bits [31:12] (that + // normally hold the pc-relative immediate) + // are used to store the lowest 20 bits of + // inst2. That is, the 12-bit immediate of + // inst2 is not included. + // + // - The location of the original inst2 is + // used to store the 32-bit absolute + // address in big endian format. Compared + // to the 20+12-bit split encoding, this + // results in a longer uninterrupted + // sequence of identical common bytes + // when the same address is referred + // with different instruction pairs + // (like AUIPC+LD vs. AUIPC+ADDI) or + // when the occurrences of the same + // pair use different registers. When + // referring to adjacent memory locations + // (like function calls that go via the + // ELF PLT), in big endian order only the + // last 1-2 bytes differ; in little endian + // the differing 1-2 bytes would be in the + // middle of the 8-byte sequence. + // + // When reversing the transformation, the + // original rd of AUIPC can be restored + // from inst2's rs1 as they are required to + // be the same. + + // Arithmetic right shift makes sign extension + // trivial but (1) it's implementation-defined + // behavior (C99/C11/C23 6.5.7-p5) and so is + // (2) casting unsigned to signed (6.3.1.3-p3). + // + // One can check for (1) with + // + // if ((-1 >> 1) == -1) ... + // + // but (2) has to be checked from the + // compiler docs. GCC promises that (1) + // and (2) behave in the common expected + // way and thus + // + // addr += (uint32_t)( + // (int32_t)inst2 >> 20); + // + // does the same as the code below. But since + // the 100 % portable way is only a few bytes + // bigger code and there is no real speed + // difference, let's just use that, especially + // since the decoder doesn't need this at all. + uint32_t addr = inst & 0xFFFFF000; + addr += (inst2 >> 20) + - ((inst2 >> 19) & 0x1000); + + addr += now_pos + (uint32_t)i; + + // Construct the first 32 bits: + // [6:0] AUIPC opcode + // [11:7] Special AUIPC rd = x2 + // [31:12] The lowest 20 bits of inst2 + inst = 0x17 | (2 << 7) | (inst2 << 12); + + write32le(buffer + i, inst); + + // The second 32 bits store the absolute + // address in big endian order. + write32be(buffer + i + 4, addr); + } else { + // AUIPC's rd equals x0 or x2. + // + // x0 indicates a landing pad (LPAD). + // It's always skipped. + // + // AUIPC with rd == x2 is used for the special + // format as explained above. When the input + // contains a byte sequence that matches the + // special format, "fake" decoding must be + // done to keep the filter bijective (that + // is, safe to apply on arbitrary data). + // + // See the "x0 or x2" section in riscv_decode() + // for how the "real" decoding is done. The + // "fake" decoding is a simplified version + // of "real" decoding with the following + // differences (these reduce code size of + // the decoder): + // (1) The lowest 12 bits aren't sign-extended. + // (2) No address conversion is done. + // (3) Big endian format isn't used (the fake + // address is in little endian order). + + // Check if inst matches the special format. + const uint32_t fake_rs1 = inst >> 27; + + if (NOT_SPECIAL_AUIPC(inst, fake_rs1)) { + i += 4 - 2; + continue; + } + + const uint32_t fake_addr = + read32le(buffer + i + 4); + + // Construct the second 32 bits: + // [19:0] Upper 20 bits from AUIPC + // [31:20] The lowest 12 bits of fake_addr + const uint32_t fake_inst2 = (inst >> 12) + | (fake_addr << 20); + + // Construct new first 32 bits from: + // [6:0] AUIPC opcode + // [11:7] Fake AUIPC rd = fake_rs1 + // [31:12] The highest 20 bits of fake_addr + inst = 0x17 | (fake_rs1 << 7) + | (fake_addr & 0xFFFFF000); + + write32le(buffer + i, inst); + write32le(buffer + i + 4, fake_inst2); + } + + i += 8 - 2; + } + } + + return i; +} + + +extern lzma_ret +lzma_simple_riscv_encoder_init(lzma_next_coder *next, + const lzma_allocator *allocator, + const lzma_filter_info *filters) +{ + return lzma_simple_coder_init(next, allocator, filters, + &riscv_encode, 0, 8, 2, true); +} +#endif + + +#ifdef HAVE_DECODER_RISCV +static size_t +riscv_decode(void *simple lzma_attribute((__unused__)), + uint32_t now_pos, + bool is_encoder lzma_attribute((__unused__)), + uint8_t *buffer, size_t size) +{ + if (size < 8) + return 0; + + size -= 8; + + size_t i; + for (i = 0; i <= size; i += 2) { + uint32_t inst = buffer[i]; + + if (inst == 0xEF) { + // JAL + const uint32_t b1 = buffer[i + 1]; + + // Only filter rd=x1(ra) and rd=x5(t0). + if ((b1 & 0x0D) != 0) + continue; + + const uint32_t b2 = buffer[i + 2]; + const uint32_t b3 = buffer[i + 3]; + const uint32_t pc = now_pos + (uint32_t)i; + +// | b3 | b2 | b1 | +// | 31 30 29 28 27 26 25 24 | 23 22 21 20 19 18 17 16 | 15 14 13 12 x x x x | +// | 20 10 9 8 7 6 5 4 | 3 2 1 11 19 18 17 16 | 15 14 13 12 x x x x | +// | 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 | 7 6 5 4 x x x x | + + uint32_t addr = ((b1 & 0xF0) << 13) + | (b2 << 9) | (b3 << 1); + + addr -= pc; + + buffer[i + 1] = (uint8_t)((b1 & 0x0F) + | ((addr >> 8) & 0xF0)); + + buffer[i + 2] = (uint8_t)(((addr >> 16) & 0x0F) + | ((addr >> 7) & 0x10) + | ((addr << 4) & 0xE0)); + + buffer[i + 3] = (uint8_t)(((addr >> 4) & 0x7F) + | ((addr >> 13) & 0x80)); + + i += 4 - 2; + + } else if ((inst & 0x7F) == 0x17) { + // AUIPC + uint32_t inst2; + + inst |= (uint32_t)buffer[i + 1] << 8; + inst |= (uint32_t)buffer[i + 2] << 16; + inst |= (uint32_t)buffer[i + 3] << 24; + + if (inst & 0xE80) { + // AUIPC's rd doesn't equal x0 or x2. + + // Check if it is a "fake" AUIPC+inst2 pair. + inst2 = read32le(buffer + i + 4); + + if (NOT_AUIPC_PAIR(inst, inst2)) { + i += 6 - 2; + continue; + } + + // Decode (or more like re-encode) the "fake" + // pair. The "fake" format doesn't do + // sign-extension, address conversion, or + // use big endian. (The use of little endian + // allows sharing the write32le() calls in + // the decoder to reduce code size when + // unaligned access isn't supported.) + uint32_t addr = inst & 0xFFFFF000; + addr += inst2 >> 20; + + inst = 0x17 | (2 << 7) | (inst2 << 12); + inst2 = addr; + } else { + // AUIPC's rd equals x0 or x2. + + // Check if inst matches the special format + // used by the encoder. + const uint32_t inst2_rs1 = inst >> 27; + + if (NOT_SPECIAL_AUIPC(inst, inst2_rs1)) { + i += 4 - 2; + continue; + } + + // Decode the "real" pair. + uint32_t addr = read32be(buffer + i + 4); + + addr -= now_pos + (uint32_t)i; + + // The second instruction: + // - Get the lowest 20 bits from inst. + // - Add the lowest 12 bits of the address + // as the immediate field. + inst2 = (inst >> 12) | (addr << 20); + + // AUIPC: + // - rd is the same as inst2_rs1. + // - The sign extension of the lowest 12 bits + // must be taken into account. + inst = 0x17 | (inst2_rs1 << 7) + | ((addr + 0x800) & 0xFFFFF000); + } + + // Both decoder branches write in little endian order. + write32le(buffer + i, inst); + write32le(buffer + i + 4, inst2); + + i += 8 - 2; + } + } + + return i; +} + + +extern lzma_ret +lzma_simple_riscv_decoder_init(lzma_next_coder *next, + const lzma_allocator *allocator, + const lzma_filter_info *filters) +{ + return lzma_simple_coder_init(next, allocator, filters, + &riscv_decode, 0, 8, 2, false); +} +#endif |