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Code generation often exposes redundant physical register copies through virtual registers such as: %vreg = COPY %PHYSREG ... %PHYSREG = COPY %vreg There are cases where no intervening clobber of %PHYSREG occurs, and the later copy could therefore be removed. In some cases this further allows us to remove the initial copy. This patch contains a motivating example which comes from the x86 build of Chrome, specifically cc::ResourceProvider::UnlockForRead uses libstdc++'s implementation of hash_map. That example has two tests live at the same time, and after machine sinking LLVM has confused itself enough and things spilling EFLAGS is a great idea even though it's never restored and the comparison results are both live. Before this patch we have: DEC32m %RIP, 1, %noreg, <ga:@L>, %noreg, %EFLAGS<imp-def> %vreg1<def> = COPY %EFLAGS; GR64:%vreg1 %EFLAGS<def> = COPY %vreg1; GR64:%vreg1 JNE_1 <BB#1>, %EFLAGS<imp-use> Both copies are useless. This patch tries to eliminate the later copy in a generic manner. dec is especially confusing to LLVM when compared with sub. I wrote this patch to treat all physical registers generically, but only remove redundant copies of non-allocatable physical registers because the allocatable ones caused issues (e.g. when calling conventions weren't properly modeled) and should be handled later by the register allocator anyways. The following tests used to failed when the patch also replaced allocatable registers: CodeGen/X86/StackColoring.ll CodeGen/X86/avx512-calling-conv.ll CodeGen/X86/copy-propagation.ll CodeGen/X86/inline-asm-fpstack.ll CodeGen/X86/musttail-varargs.ll CodeGen/X86/pop-stack-cleanup.ll CodeGen/X86/preserve_mostcc64.ll CodeGen/X86/tailcallstack64.ll CodeGen/X86/this-return-64.ll This happens because COPY has other special meaning for e.g. dependency breakage and x87 FP stack. Note that all other backends' tests pass. Reviewers: qcolombet Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D15157 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254665 91177308-0d34-0410-b5e6-96231b3b80d8
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