[PATCH] D153838: [MCP] Optimize copies from undef

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh created this revision.
Pierre-vh added reviewers: arsenm, foad, critson, qcolombet.
Herald added subscribers: StephenFan, pengfei, hiraditya.
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Pierre-vh requested review of this revision.
Herald added subscribers: llvm-commits, wdng.
Herald added a project: LLVM.

Revert D152502 <https://reviews.llvm.org/D152502> and instead optimize away copy from undefs, but clear the undef flag on the original copy.
Apparently, not optimizing the COPY can cause performance issues in some cases.

Fixes SWDEV-405813, SWDEV-405899


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D153838

Files:
  llvm/lib/CodeGen/MachineCopyPropagation.cpp
  llvm/test/CodeGen/X86/machine-copy-prop.mir


Index: llvm/test/CodeGen/X86/machine-copy-prop.mir
===================================================================
--- llvm/test/CodeGen/X86/machine-copy-prop.mir
+++ llvm/test/CodeGen/X86/machine-copy-prop.mir
@@ -220,9 +220,8 @@
 # second COPY, otherwise we risk making $rax undef.
 # CHECK-LABEL: name: nocopyprop6
 # CHECK: bb.0:
-# CHECK:      $rax = COPY undef $rdi
-# CHECK-NEXT: NOOP implicit killed $rax
-# CHECK-NEXT: $rax = COPY $rdi
+# CHECK:      $rax = COPY $rdi
+# CHECK-NEXT: NOOP implicit $rax
 # CHECK-NEXT: NOOP implicit $rax, implicit $rdi
 name: nocopyprop6
 tracksRegLiveness: true
Index: llvm/lib/CodeGen/MachineCopyPropagation.cpp
===================================================================
--- llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -462,8 +462,7 @@
 
   auto PrevCopyOperands = isCopyInstr(*PrevCopy, *TII, UseCopyInstr);
   // Check that the existing copy uses the correct sub registers.
-  if (PrevCopyOperands->Destination->isDead() ||
-      PrevCopyOperands->Source->isUndef())
+  if (PrevCopyOperands->Destination->isDead())
     return false;
   if (!isNopCopy(*PrevCopy, Src, Def, TRI, TII, UseCopyInstr))
     return false;
@@ -482,6 +481,12 @@
        make_range(PrevCopy->getIterator(), Copy.getIterator()))
     MI.clearRegisterKills(CopyDef, TRI);
 
+  // Clear undef flag from remaining copy if needed.
+  if (!CopyOperands->Source->isUndef()) {
+    PrevCopy->getOperand(PrevCopyOperands->Source->getOperandNo())
+        .setIsUndef(false);
+  }
+
   Copy.eraseFromParent();
   Changed = true;
   ++NumDeletes;


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