[GlobalISel] Avoid invalidating ValToVReg when translating no-op bitcast.

When we translate a no-op (same type) bitcast, we try to be clever and
only emit a COPY if we already assigned a vreg to the defined value.
However, when we didn't, we tried to assign to a reference into the
ValToVReg DenseMap, even though the RHS of the assignment
(getOrCreateVReg) could potentially grow that DenseMap, invalidating the
reference.

Avoid that by getting the source vreg first.
I audited the rest of the translator; this is the only tricky case.

The test is quite unwieldy, as the problem is caused by the DenseMap
growing, which happens after the 47th mapped value.

llvm-svn: 297208

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 70333057a76..1a70947a4aa 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -396,12 +396,17 @@ bool IRTranslator::translateSelect(const User &U,
 
 bool IRTranslator::translateBitCast(const User &U,
                                     MachineIRBuilder &MIRBuilder) {
+  // If we're bitcasting to the source type, we can reuse the source vreg.
   if (LLT{*U.getOperand(0)->getType(), *DL} == LLT{*U.getType(), *DL}) {
+    // Get the source vreg now, to avoid invalidating ValToVReg.
+    unsigned SrcReg = getOrCreateVReg(*U.getOperand(0));
     unsigned &Reg = ValToVReg[&U];
+    // If we already assigned a vreg for this bitcast, we can't change that.
+    // Emit a copy to satisfy the users we already emitted.
     if (Reg)
-      MIRBuilder.buildCopy(Reg, getOrCreateVReg(*U.getOperand(0)));
+      MIRBuilder.buildCopy(Reg, SrcReg);
     else
-      Reg = getOrCreateVReg(*U.getOperand(0));
+      Reg = SrcReg;
     return true;
   }
   return translateCast(TargetOpcode::G_BITCAST, U, MIRBuilder);