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Dynamically realign local variables with large alignment

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This works around the lack of a way to specify the alignment of a stack
slot in Cranelift.

Fixes #1230
Fixes #1381
This commit is contained in:
bjorn3 2023-10-22 17:11:11 +00:00
parent 38e8be9673
commit 55380a5ffd
2 changed files with 33 additions and 7 deletions
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11
example/mini_core_hello_world.rs
View File

@ -353,6 +353,17 @@ fn main() {
let f = V([0.0, 1.0]);
let _a = f.0[0];
stack_val_align();
}
#[inline(never)]
fn stack_val_align() {
#[repr(align(8192))]
struct Foo(u8);
let a = Foo(0);
assert_eq!(&a as *const Foo as usize % 8192, 0);
}
#[cfg(all(

29
src/common.rs
View File

@ -384,13 +384,28 @@ impl<'tcx> FunctionCx<'_, '_, 'tcx> {
}
pub(crate) fn create_stack_slot(&mut self, size: u32, align: u32) -> Pointer {
let stack_slot = self.bcx.create_sized_stack_slot(StackSlotData {
kind: StackSlotKind::ExplicitSlot,
// FIXME Don't force the size to a multiple of 16 bytes once Cranelift gets a way to
// specify stack slot alignment.
size: (size + 15) / 16 * 16,
});
Pointer::stack_slot(stack_slot)
if align <= 16 {
let stack_slot = self.bcx.create_sized_stack_slot(StackSlotData {
kind: StackSlotKind::ExplicitSlot,
// FIXME Don't force the size to a multiple of 16 bytes once Cranelift gets a way to
// specify stack slot alignment.
size: (size + 15) / 16 * 16,
});
Pointer::stack_slot(stack_slot)
} else {
// Alignment is too big to handle using the above hack. 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 is this calculation to ensure there is enough space to dyanmically realign
// as well as keep a 16 byte realignment for the other stack slots correct?
size: ((size + align - 1) + 16) / 16 * 16,
});
let base_ptr = self.bcx.ins().stack_addr(self.pointer_type, stack_slot, 0);
let misalign_offset = self.bcx.ins().urem_imm(base_ptr, i64::from(align));
let realign_offset = self.bcx.ins().irsub_imm(misalign_offset, i64::from(align));
Pointer::new(self.bcx.ins().iadd(base_ptr, realign_offset))
}
}
pub(crate) fn set_debug_loc(&mut self, source_info: mir::SourceInfo) {