[x86] Fix PR37100 by teaching the EFLAGS copy lowering to rewrite uses

across basic blocks in the limited cases where it is very straight
forward to do so.

This will also be useful for other places where we do some limited
EFLAGS propagation across CFG edges and need to handle copy rewrites
afterward. I think this is rapidly approaching the maximum we can and
should be doing here. Everything else begins to require either heroic
analysis to prove how to do PHI insertion manually, or somehow managing
arbitrary PHI-ing of EFLAGS with general PHI insertion. Neither of these
seem at all promising so if those cases come up, we'll almost certainly
need to rewrite the parts of LLVM that produce those patterns.

We do now require dominator trees in order to reliably diagnose patterns
that would require PHI nodes. This is a bit unfortunate but it seems
better than the completely mysterious crash we would get otherwise.

Differential Revision: https://reviews.llvm.org/D45673

llvm-svn: 330264

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/O3-pipeline.ll b/llvm/test/CodeGen/X86/O3-pipeline.ll
index 01deba7eed1..8511474d8f4 100644
--- a/llvm/test/CodeGen/X86/O3-pipeline.ll
+++ b/llvm/test/CodeGen/X86/O3-pipeline.ll
@@ -90,6 +90,7 @@
 ; CHECK-NEXT:       X86 LEA Optimize
 ; CHECK-NEXT:       X86 Optimize Call Frame
 ; CHECK-NEXT:       X86 Avoid Store Forwarding Block
+; CHECK-NEXT:       MachineDominator Tree Construction
 ; CHECK-NEXT:       X86 EFLAGS copy lowering
 ; CHECK-NEXT:       X86 WinAlloca Expander
 ; CHECK-NEXT:       Detect Dead Lanes