0becc89d4a
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
36 lines
500 B
C
36 lines
500 B
C
#include <assert.h>
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#include <stdbool.h>
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#include <stdint.h>
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#include <string.h>
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struct TwoU64s
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{
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uint64_t a;
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uint64_t b;
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} __attribute__((aligned(16)));
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struct BoolAndU32
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{
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bool a;
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uint32_t b;
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};
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int32_t many_args(
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void *a,
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void *b,
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const char *c,
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uint64_t d,
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bool e,
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struct BoolAndU32 f,
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void *g,
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struct TwoU64s h,
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void *i,
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void *j,
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void *k,
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void *l,
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const char *m)
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{
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assert(strcmp(m, "Hello world") == 0);
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return 0;
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}
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