[llvm] [InstCombine] Implement processSMulSExtIdiom for Signed Multiplication Overflow Detection (PR #131461)

[via llvm-commits]

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@@ -6526,6 +6526,172 @@ static Instruction *processUMulZExtIdiom(ICmpInst &I, Value *MulVal,
   return ExtractValueInst::Create(Call, 1);
 }
 
+/// Recognize and process idiom involving test for multiplication
+/// overflow.
+///
+/// The caller has matched a pattern of the form:
+///   I = cmp u add (mul(sext A, sext B), V, W
+/// The function checks if this is a test for overflow and if so replaces
+/// multiplication with call to 'mul.with.overflow' intrinsic.
+///
+/// \param I Compare instruction.
+/// \param MulVal Result of 'mult' instruction.  It is one of the arguments of
+///               the compare instruction.  Must be of integer type.
+/// \param OtherVal The other argument of compare instruction.
+/// \returns Instruction which must replace the compare instruction, NULL if no
+///          replacement required.
+static Instruction *processSMulSExtIdiom(ICmpInst &I, Value *MulVal,
+                                         const APInt *AddVal,
+                                         const APInt *OtherVal,
+                                         InstCombinerImpl &IC) {
+  // Don't bother doing this transformation for pointers, don't do it for
+  // vectors.
+  if (!isa<IntegerType>(MulVal->getType()))
+    return nullptr;
+
+  auto *MulInstr = dyn_cast<Instruction>(MulVal);
+  if (!MulInstr)
+    return nullptr;
+  assert(MulInstr->getOpcode() == Instruction::Mul);
+
+  auto *LHS = cast<SExtInst>(MulInstr->getOperand(0)),
+       *RHS = cast<SExtInst>(MulInstr->getOperand(1));
+  assert(LHS->getOpcode() == Instruction::SExt);
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DTeachs wrote:

> These asserts seem unnecessary.

Copy pasted from the processUMulZExtIdiom, which checks for Instruction::ZExt, so I am iffy on removing them but at the same time, I understand why they are not needed. As a first time contributor, I am nervous to remove them. 

https://github.com/llvm/llvm-project/pull/131461