bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

rustc_target: Add alignment to indirectly-passed by-value types, correcting the

Browse source 
alignment of `byval` on x86 in the process.

Commit 88e4d2c291 from five years ago removed
support for alignment on indirectly-passed arguments because of problems with
the `i686-pc-windows-msvc` target. Unfortunately, the `memcpy` optimizations I
recently added to LLVM 16 depend on this to forward `memcpy`s. This commit
attempts to fix the problems with `byval` parameters on that target and now
correctly adds the `align` attribute.

The problem is summarized in [this comment] by @eddyb. Briefly, 32-bit x86 has
special alignment rules for `byval` parameters: for the most part, their
alignment is forced to 4. This is not well-documented anywhere but in the Clang
source. I looked at the logic in Clang `TargetInfo.cpp` and tried to replicate
it here. The relevant methods in that file are
`X86_32ABIInfo::getIndirectResult()` and
`X86_32ABIInfo::getTypeStackAlignInBytes()`. The `align` parameter attribute
for `byval` parameters in LLVM must match the platform ABI, or miscompilations
will occur. Note that this doesn't use the approach suggested by eddyb, because
I felt it was overkill to store the alignment in `on_stack` when special
handling is really only needed for 32-bit x86.

As a side effect, this should fix #80127, because it will make the `align`
parameter attribute for `byval` parameters match the platform ABI on LLVM
x86-64.

[this comment]: https://github.com/rust-lang/rust/pull/80822#issuecomment-829985417
This commit is contained in:
Patrick Walton 2022-10-31 20:38:40 -07:00 • committed by Erik Desjardins
parent 8ca44ef9ca
commit 0becc89d4a
11 changed files with 208 additions and 17 deletions
Download patch file Download diff file Expand all files Collapse all files

4
tests/codegen/function-arguments-noopt.rs
View file

@ -42,7 +42,7 @@ pub fn borrow_call(x: &i32, f: fn(&i32) -> &i32) -> &i32 {
f(x)
}
// CHECK: void @struct_({{%S\*|ptr}} sret(%S){{( %_0)?}}, {{%S\*|ptr}} %x)
// CHECK: void @struct_({{%S\*|ptr}} sret(%S) align 4{{( %_0)?}}, {{%S\*|ptr}} align 4 %x)
#[no_mangle]
pub fn struct_(x: S) -> S {
x
@ -51,7 +51,7 @@ pub fn struct_(x: S) -> S {
// CHECK-LABEL: @struct_call
#[no_mangle]
pub fn struct_call(x: S, f: fn(S) -> S) -> S {
// CHECK: call void %f({{%S\*|ptr}} sret(%S){{( %_0)?}}, {{%S\*|ptr}} %{{.+}})
// CHECK: call void %f({{%S\*|ptr}} sret(%S) align 4{{( %_0)?}}, {{%S\*|ptr}} align 4 %{{.+}})
f(x)
}