[ARM] Disallow icmp with negative imm and overflow

We allow overflowing instructions if they're decreasing and only used
by an unsigned compare. Add the extra condition that the icmp cannot
be using a negative immediate.

Differential Revision: https://reviews.llvm.org/D52102

llvm-svn: 342392

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMCodeGenPrepare.cpp b/llvm/lib/Target/ARM/ARMCodeGenPrepare.cpp
index e06f05a2d98..a1241b7a1e2 100644
--- a/llvm/lib/Target/ARM/ARMCodeGenPrepare.cpp
+++ b/llvm/lib/Target/ARM/ARMCodeGenPrepare.cpp
@@ -257,9 +257,20 @@ static bool isSafeOverflow(Instruction *I) {
       return false;
 
     auto *CI = cast<ICmpInst>(*I->user_begin());
+    
+    // Don't support an icmp that deals with sign bits, including negative
+    // immediates
     if (CI->isSigned())
       return false;
 
+    if (auto *Const = dyn_cast<ConstantInt>(CI->getOperand(0)))
+      if (Const->isNegative())
+        return false;
+
+    if (auto *Const = dyn_cast<ConstantInt>(CI->getOperand(1)))
+      if (Const->isNegative())
+        return false;
+
     bool NegImm = cast<ConstantInt>(I->getOperand(1))->isNegative();
     bool IsDecreasing = ((Opc == Instruction::Sub) && !NegImm) ||
                         ((Opc == Instruction::Add) && NegImm);