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Revert "rustc: Fix (again) simd vectors by-val in ABI"

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This reverts commit 3cc8f738d4.
This commit is contained in:
Alex Crichton 2018-10-23 00:59:14 -07:00
parent d570b36cd9
commit 086f5a55be
9 changed files with 9 additions and 332 deletions
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34
src/librustc_codegen_llvm/back/write.rs
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@ -633,7 +633,7 @@ unsafe fn optimize(cgcx: &CodegenContext,
None,
&format!("llvm module passes [{}]", module_name.unwrap()),
|| {
llvm::LLVMRunPassManager(mpm, llmod);
llvm::LLVMRunPassManager(mpm, llmod)
});
// Deallocate managers that we're now done with
@ -691,38 +691,6 @@ unsafe fn codegen(cgcx: &CodegenContext,
create_msvc_imps(cgcx, llcx, llmod);
}
// Ok now this one's a super interesting invocations. SIMD in rustc is
// difficult where we want some parts of the program to be able to use
// some SIMD features while other parts of the program don't. The real
// tough part is that we want this to actually work correctly!
//
// We go to great lengths to make sure this works, and one crucial
// aspect is that vector arguments (simd types) are never passed by
// value in the ABI of functions. It turns out, however, that LLVM will
// undo our "clever work" of passing vector types by reference. Its
// argument promotion pass will promote these by-ref arguments to
// by-val. That, however, introduces codegen errors!
//
// The upstream LLVM bug [1] has unfortunatey not really seen a lot of
// activity. The Rust bug [2], however, has seen quite a lot of reports
// of this in the wild. As a result, this is worked around locally here.
// We have a custom transformation, `LLVMRustDemoteSimdArguments`, which
// does the opposite of argument promotion by demoting any by-value SIMD
// arguments in function signatures to pointers intead of being
// by-value.
//
// This operates at the LLVM IR layer because LLVM is thwarting our
// codegen and this is the only chance we get to make sure it's correct
// before we hit codegen.
//
// Hopefully one day the upstream LLVM bug will be fixed and we'll no
// longer need this!
//
// [1]: https://bugs.llvm.org/show_bug.cgi?id=37358
// [2]: https://github.com/rust-lang/rust/issues/50154
llvm::LLVMRustDemoteSimdArguments(llmod);
cgcx.save_temp_bitcode(&module, "simd-demoted");
// A codegen-specific pass manager is used to generate object
// files for an LLVM module.
//