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4abd22ba6b
Reapply r242295 with fixes in the implementation. - Teaches the ValueTracker in the PeepholeOptimizer to look through PHI instructions. - Add findNextSourceAndRewritePHI method to lookup into multiple sources returnted by the ValueTracker and rewrite PHIs with new sources. With these changes we can find more register sources and rewrite more copies to allow coaslescing of bitcast instructions. Hence, we eliminate unnecessary VR64 <-> GR64 copies in x86, but it could be extended to other archs by marking "isBitcast" on target specific instructions. The x86 example follows: A: psllq %mm1, %mm0 movd %mm0, %r9 jmp C B: por %mm1, %mm0 movd %mm0, %r9 jmp C C: movd %r9, %mm0 pshufw $238, %mm0, %mm0 Becomes: A: psllq %mm1, %mm0 jmp C B: por %mm1, %mm0 jmp C C: pshufw $238, %mm0, %mm0 Differential Revision: http://reviews.llvm.org/D11197 rdar://problem/20404526 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243271 91177308-0d34-0410-b5e6-96231b3b80d8
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