InstCombine, InstSimplify: (%X /s C1) /s C2 isn't always 0 when C1 * C2 overflow

consider:
C1 = INT_MIN
C2 = -1

C1 * C2 overflows without a doubt but consider the following:
%x = i32 INT_MIN

This means that (%X /s C1) is 1 and (%X /s C1) /s C2 is -1.

N. B.  Move the unsigned version of this transform to InstSimplify, it
doesn't create any new instructions.

This fixes PR21243.

llvm-svn: 219567

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index f2f67067d15..a01bac2077f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1057,6 +1057,16 @@ static Value *SimplifyDiv(Instruction::BinaryOps Opcode, Value *Op0, Value *Op1,
       (!isSigned && match(Op0, m_URem(m_Value(), m_Specific(Op1)))))
     return Constant::getNullValue(Op0->getType());
 
+  // (X /u C1) /u C2 -> 0 if C1 * C2 overflow
+  ConstantInt *C1, *C2;
+  if (!isSigned && match(Op0, m_UDiv(m_Value(X), m_ConstantInt(C1))) &&
+      match(Op1, m_ConstantInt(C2))) {
+    bool Overflow;
+    C1->getValue().umul_ov(C2->getValue(), Overflow);
+    if (Overflow)
+      return Constant::getNullValue(Op0->getType());
+  }
+
   // If the operation is with the result of a select instruction, check whether
   // operating on either branch of the select always yields the same value.
   if (isa<SelectInst>(Op0) || isa<SelectInst>(Op1))