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Only allocate the minimal alignment requested for a type when possible. (#1604)

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* Only allocate the minimal alignment requested for a type when possible.

* Code review: make an assertion a debug assert. Adjust an out-of-date comment.
This commit is contained in:
srosefield-riverside
2025-11-17 09:31:02 -05:00
committed by GitHub
parent 614349ab2c
commit 726b7360cf
1 changed files with 9 additions and 9 deletions
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src/common.rs
+9 -9
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@@ -377,26 +377,26 @@ pub(crate) fn create_stack_slot(&mut self, size: u32, align: u32) -> Pointer {
size.is_multiple_of(align),
"size must be a multiple of alignment (size={size}, align={align})"
);
debug_assert!(align.is_power_of_two(), "alignment must be a power of two (align={align})");
let abi_align = if self.tcx.sess.target.arch == Arch::S390x { 8 } else { 16 };
// Cranelift can only guarantee alignment up to the ABI alignment provided by the target.
// If the requested alignment is less than the abi_align it can be used directly.
if align <= abi_align {
let stack_slot = self.bcx.create_sized_stack_slot(StackSlotData {
kind: StackSlotKind::ExplicitSlot,
// FIXME Don't force the size to a multiple of <abi_align> bytes once Cranelift gets
// a way to specify stack slot alignment.
size: size.div_ceil(abi_align) * abi_align,
align_shift: 4,
size,
// The maximum value of ilog2 is 31 which will always fit in a u8.
align_shift: align.ilog2().try_into().unwrap(),
});
Pointer::stack_slot(stack_slot)
} else {
// Alignment is too big to handle using the above hack. Dynamically realign a stack slot
// Alignment is larger than the ABI alignment guaranteed. Dynamically realign a stack slot
// instead. This wastes some space for the realignment.
let stack_slot = self.bcx.create_sized_stack_slot(StackSlotData {
kind: StackSlotKind::ExplicitSlot,
// FIXME Don't force the size to a multiple of <abi_align> bytes once Cranelift gets
// a way to specify stack slot alignment.
size: (size + align) / abi_align * abi_align,
align_shift: 4,
size: size + align,
align_shift: abi_align.ilog2().try_into().unwrap(),
});
let base_ptr = self.bcx.ins().stack_addr(self.pointer_type, stack_slot, 0);
let misalign_offset = self.bcx.ins().band_imm(base_ptr, i64::from(align - 1));