[globalisel][combines] Don't sink G_TRUNC down to use if that use is a G_PHI

This fixes a problem where the register allocator fails to eliminate a PHI
because there's a non-PHI in the middle of the PHI instructions at the start
of a BB.

This G_TRUNC can be better placed but this at least fixes the correctness issue
quickly. I'll follow up with a patch to the verifier to catch this kind of bug
in future.

llvm-svn: 343693

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index 98114d295a1..30e0da319e8 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -154,12 +154,18 @@ bool CombinerHelper::tryCombineExtendingLoads(MachineInstr &MI) {
   assert(Preferred.Ty != LoadValueTy && "Extending to same type?");
 
   // Rewrite the load and schedule the canonical use for erasure.
-  const auto TruncateUse = [](MachineIRBuilder &Builder, MachineOperand &UseMO,
-                              unsigned DstReg, unsigned SrcReg) {
+  const auto TruncateUse = [&MI](MachineIRBuilder &Builder,
+                                 MachineOperand &UseMO, unsigned DstReg,
+                                 unsigned SrcReg) {
     MachineInstr &UseMI = *UseMO.getParent();
     MachineBasicBlock &UseMBB = *UseMI.getParent();
 
     Builder.setInsertPt(UseMBB, MachineBasicBlock::iterator(UseMI));
+
+    if (UseMI.isPHI())
+      Builder.setInsertPt(*MI.getParent(),
+                          std::next(MachineBasicBlock::iterator(MI)));
+
     Builder.buildTrunc(DstReg, SrcReg);
   };