PeepholeOptimizer: Do not replace SubregToReg(bitcast like)
While we can usually replace bitcast like instructions
(MachineInstr::isBitcast()) with a COPY this is not legal if any of the
users uses SUBREG_TO_REG to assert the upper bits of the result are
zero.
Differential Revision: https://reviews.llvm.org/D28474
llvm-svn: 291483
diff --git a/llvm/lib/CodeGen/PeepholeOptimizer.cpp b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
index 11af50f..6d64345 100644
--- a/llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -1715,7 +1715,8 @@
// Bitcasts with more than one def are not supported.
if (Def->getDesc().getNumDefs() != 1)
return ValueTrackerResult();
- if (Def->getOperand(DefIdx).getSubReg() != DefSubReg)
+ const MachineOperand DefOp = Def->getOperand(DefIdx);
+ if (DefOp.getSubReg() != DefSubReg)
// If we look for a different subreg, it means we want a subreg of the src.
// Bails as we do not support composing subregs yet.
return ValueTrackerResult();
@@ -1735,6 +1736,14 @@
return ValueTrackerResult();
SrcIdx = OpIdx;
}
+
+ // Stop when any user of the bitcast is a SUBREG_TO_REG, replacing with a COPY
+ // will break the assumed guarantees for the upper bits.
+ for (const MachineInstr &UseMI : MRI.use_nodbg_instructions(DefOp.getReg())) {
+ if (UseMI.isSubregToReg())
+ return ValueTrackerResult();
+ }
+
const MachineOperand &Src = Def->getOperand(SrcIdx);
return ValueTrackerResult(Src.getReg(), Src.getSubReg());
}