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0becc89d4a
alignment of `byval` on x86 in the process. Commit 88e4d2c2918428d55e34cd57c11279ea839c8822 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
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