1 files changed, 241 insertions, 0 deletions
diff --git a/third_party/rust/cranelift-codegen/src/stack_layout.rs b/third_party/rust/cranelift-codegen/src/stack_layout.rs
new file mode 100644
index 0000000000..2430e8a643
--- /dev/null
+++ b/third_party/rust/cranelift-codegen/src/stack_layout.rs
@@ -0,0 +1,241 @@
+//! Computing stack layout.
+
+use crate::ir::stackslot::{StackOffset, StackSize, StackSlotKind};
+use crate::ir::{StackLayoutInfo, StackSlots};
+use crate::result::{CodegenError, CodegenResult};
+use core::cmp::{max, min};
+
+/// Compute the stack frame layout.
+///
+/// Determine the total size of this stack frame and assign offsets to all `Spill` and `Explicit`
+/// stack slots.
+///
+/// The total frame size will be a multiple of `alignment` which must be a power of two, unless the
+/// function doesn't perform any call.
+///
+/// Returns the total stack frame size which is also saved in `frame.frame_size`.
+///
+/// If the stack frame is too big, returns an `ImplLimitExceeded` error.
+pub fn layout_stack(
+    frame: &mut StackSlots,
+    is_leaf: bool,
+    alignment: StackSize,
+) -> CodegenResult<StackSize> {
+    // Each object and the whole stack frame must fit in 2 GB such that any relative offset within
+    // the frame fits in a `StackOffset`.
+    let max_size = StackOffset::max_value() as StackSize;
+    debug_assert!(alignment.is_power_of_two() && alignment <= max_size);
+
+    // We assume a stack that grows toward lower addresses as implemented by modern ISAs. The
+    // stack layout from high to low addresses will be:
+    //
+    // 1. incoming arguments.
+    // 2. spills + explicits + struct returns.
+    // 3. outgoing arguments.
+    //
+    // The incoming arguments can have both positive and negative offsets. A negative offset
+    // incoming arguments is usually the x86 return address pushed by the call instruction, but
+    // it can also be fixed stack slots pushed by an externally generated prologue.
+    //
+    // Both incoming and outgoing argument slots have fixed offsets that are treated as
+    // reserved zones by the layout algorithm.
+    //
+    // If a function only has incoming arguments and does not perform any calls, then it doesn't
+    // require the stack to be aligned.
+
+    let mut incoming_min = 0;
+    let mut incoming_max = 0;
+    let mut outgoing_max = 0;
+    let mut min_align = alignment;
+    let mut must_align = !is_leaf;
+
+    for slot in frame.values() {
+        if slot.size > max_size {
+            return Err(CodegenError::ImplLimitExceeded);
+        }
+
+        match slot.kind {
+            StackSlotKind::IncomingArg => {
+                incoming_min = min(incoming_min, slot.offset.unwrap());
+                incoming_max = max(incoming_max, slot.offset.unwrap() + slot.size as i32);
+            }
+            StackSlotKind::OutgoingArg => {
+                let offset = slot
+                    .offset
+                    .unwrap()
+                    .checked_add(slot.size as StackOffset)
+                    .ok_or(CodegenError::ImplLimitExceeded)?;
+                outgoing_max = max(outgoing_max, offset);
+                must_align = true;
+            }
+            StackSlotKind::StructReturnSlot
+            | StackSlotKind::SpillSlot
+            | StackSlotKind::ExplicitSlot
+            | StackSlotKind::EmergencySlot => {
+                // Determine the smallest alignment of any explicit or spill slot.
+                min_align = slot.alignment(min_align);
+                must_align = true;
+            }
+        }
+    }
+
+    // Lay out spill slots, struct return slots, and explicit slots below the
+    // incoming arguments. The offset is negative, growing downwards. Start with
+    // the smallest alignments for better packing.
+    let mut offset = incoming_min;
+    debug_assert!(min_align.is_power_of_two());
+    while min_align <= alignment {
+        for slot in frame.values_mut() {
+            // Pick out explicit and spill slots with exact alignment `min_align`.
+            match slot.kind {
+                StackSlotKind::SpillSlot
+                | StackSlotKind::StructReturnSlot
+                | StackSlotKind::ExplicitSlot
+                | StackSlotKind::EmergencySlot => {
+                    if slot.alignment(alignment) != min_align {
+                        continue;
+                    }
+                }
+                StackSlotKind::IncomingArg | StackSlotKind::OutgoingArg => continue,
+            }
+
+            offset = offset
+                .checked_sub(slot.size as StackOffset)
+                .ok_or(CodegenError::ImplLimitExceeded)?;
+
+            // Aligning the negative offset can never cause overflow. We're only clearing bits.
+            offset &= -(min_align as StackOffset);
+            slot.offset = Some(offset);
+        }
+
+        // Move on to the next higher alignment.
+        min_align *= 2;
+    }
+
+    // Finally, make room for the outgoing arguments.
+    offset = offset
+        .checked_sub(outgoing_max)
+        .ok_or(CodegenError::ImplLimitExceeded)?;
+
+    if must_align {
+        offset &= -(alignment as StackOffset);
+    }
+
+    // Set the computed layout information for the frame
+    let frame_size = (offset as StackSize).wrapping_neg();
+    let inbound_args_size = incoming_max as u32;
+    frame.layout_info = Some(StackLayoutInfo {
+        frame_size,
+        inbound_args_size,
+    });
+
+    Ok(frame_size)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::layout_stack;
+    use crate::ir::stackslot::StackOffset;
+    use crate::ir::types;
+    use crate::ir::{StackSlotData, StackSlotKind, StackSlots};
+    use crate::result::CodegenError;
+
+    #[test]
+    fn layout() {
+        let sss = &mut StackSlots::new();
+
+        // For all these test cases, assume it will call.
+        let is_leaf = false;
+
+        // An empty layout should have 0-sized stack frame.
+        assert_eq!(layout_stack(sss, is_leaf, 1), Ok(0));
+        assert_eq!(layout_stack(sss, is_leaf, 16), Ok(0));
+
+        // Same for incoming arguments with non-negative offsets.
+        let in0 = sss.make_incoming_arg(8, 0);
+        let in1 = sss.make_incoming_arg(8, 8);
+
+        assert_eq!(layout_stack(sss, is_leaf, 1), Ok(0));
+        assert_eq!(layout_stack(sss, is_leaf, 16), Ok(0));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+
+        // Add some spill slots.
+        let ss0 = sss.make_spill_slot(types::I64);
+        let ss1 = sss.make_spill_slot(types::I32);
+
+        assert_eq!(layout_stack(sss, is_leaf, 1), Ok(12));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[ss0].offset, Some(-8));
+        assert_eq!(sss[ss1].offset, Some(-12));
+
+        assert_eq!(layout_stack(sss, is_leaf, 16), Ok(16));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[ss0].offset, Some(-16));
+        assert_eq!(sss[ss1].offset, Some(-4));
+
+        // An incoming argument with negative offset counts towards the total frame size, but it
+        // should still pack nicely with the spill slots.
+        let in2 = sss.make_incoming_arg(4, -4);
+
+        assert_eq!(layout_stack(sss, is_leaf, 1), Ok(16));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[in2].offset, Some(-4));
+        assert_eq!(sss[ss0].offset, Some(-12));
+        assert_eq!(sss[ss1].offset, Some(-16));
+
+        assert_eq!(layout_stack(sss, is_leaf, 16), Ok(16));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[in2].offset, Some(-4));
+        assert_eq!(sss[ss0].offset, Some(-16));
+        assert_eq!(sss[ss1].offset, Some(-8));
+
+        // Finally, make sure there is room for the outgoing args.
+        let out0 = sss.get_outgoing_arg(4, 0);
+
+        assert_eq!(layout_stack(sss, is_leaf, 1), Ok(20));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[in2].offset, Some(-4));
+        assert_eq!(sss[ss0].offset, Some(-12));
+        assert_eq!(sss[ss1].offset, Some(-16));
+        assert_eq!(sss[out0].offset, Some(0));
+
+        assert_eq!(layout_stack(sss, is_leaf, 16), Ok(32));
+        assert_eq!(sss[in0].offset, Some(0));
+        assert_eq!(sss[in1].offset, Some(8));
+        assert_eq!(sss[in2].offset, Some(-4));
+        assert_eq!(sss[ss0].offset, Some(-16));
+        assert_eq!(sss[ss1].offset, Some(-8));
+        assert_eq!(sss[out0].offset, Some(0));
+
+        // Also test that an unsupported offset is rejected.
+        sss.get_outgoing_arg(1, StackOffset::max_value() - 1);
+        assert_eq!(
+            layout_stack(sss, is_leaf, 1),
+            Err(CodegenError::ImplLimitExceeded)
+        );
+    }
+
+    #[test]
+    fn slot_kinds() {
+        let sss = &mut StackSlots::new();
+
+        // Add some slots of various kinds.
+        let ss0 = sss.make_spill_slot(types::I32);
+        let ss1 = sss.push(StackSlotData::new(
+            StackSlotKind::ExplicitSlot,
+            types::I32.bytes(),
+        ));
+        let ss2 = sss.get_emergency_slot(types::I32, &[]);
+
+        assert_eq!(layout_stack(sss, true, 1), Ok(12));
+        assert_eq!(sss[ss0].offset, Some(-4));
+        assert_eq!(sss[ss1].offset, Some(-8));
+        assert_eq!(sss[ss2].offset, Some(-12));
+    }
+}