[ARM] Check that CPSR does not have other uses

Fix up rL356335 by checking that CPSR is not read between
the compare and the branch.

llvm-svn: 356349

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
index 7aefa01f63d..429c2a582bb 100644
--- a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -1912,13 +1912,17 @@ bool ARMConstantIslands::optimizeThumb2Branches() {
     if (BrOffset >= DestOffset || (DestOffset - BrOffset) > 126)
       continue;
 
-    // Search backwards to the instruction that defines CSPR
+    // Search backwards to the instruction that defines CSPR. This may or not
+    // be a CMP, we check that after this loop. If we find an instruction that
+    // reads cpsr, we need to keep the original cmp.
     auto *TRI = STI->getRegisterInfo();
     MachineBasicBlock::iterator CmpMI = Br.MI;
     while (CmpMI != Br.MI->getParent()->begin()) {
       --CmpMI;
       if (CmpMI->modifiesRegister(ARM::CPSR, TRI))
         break;
+      if (CmpMI->readsRegister(ARM::CPSR, TRI))
+        break;
     }
 
     // Check that this inst is a CMP r[0-7], #0 and that the register