[IR] `SCEVExpander::generateOverflowCheck()`: short-circuit `umul_with_overflow`-by-one

It's a no-op, no overflow happens ever: https://alive2.llvm.org/ce/z/Zw89rZ

While generally i don't like such hacks,
we have a very good reason to do this: here we are expanding
a run-time correctness check for the vectorization,
and said `umul_with_overflow` will not be optimized out
before we query the cost of the checks we've generated.

Which means, the cost of run-time checks would be artificially inflated,
and after https://reviews.llvm.org/D109368 that will affect
the minimal trip count for which these checks are even evaluated.
And if they aren't even evaluated, then the vectorized code
certainly won't be run.

We could consider doing this in IRBuilder,  but then we'd need to
also teach `CreateExtractValue()` to look into chain of `insertvalue`'s,
and i'm not sure there's precedent for that.

Refs. https://reviews.llvm.org/D109368#3089809

1 files changed, 14 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 252eea6..3abd189 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -2494,13 +2494,22 @@ Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
 
   // Get the backedge taken count and truncate or extended to the AR type.
   Value *TruncTripCount = Builder.CreateZExtOrTrunc(TripCountVal, Ty);
-  auto *MulF = Intrinsic::getDeclaration(Loc->getModule(),
-                                         Intrinsic::umul_with_overflow, Ty);
 
   // Compute |Step| * Backedge
-  CallInst *Mul = Builder.CreateCall(MulF, {AbsStep, TruncTripCount}, "mul");
-  Value *MulV = Builder.CreateExtractValue(Mul, 0, "mul.result");
-  Value *OfMul = Builder.CreateExtractValue(Mul, 1, "mul.overflow");
+  Value *MulV, *OfMul;
+  if (Step->isOne()) {
+    // Special-case Step of one. Potentially-costly `umul_with_overflow` isn't
+    // needed, there is never an overflow, so to avoid artificially inflating
+    // the cost of the check, directly emit the optimized IR.
+    MulV = TruncTripCount;
+    OfMul = ConstantInt::getFalse(MulV->getContext());
+  } else {
+    auto *MulF = Intrinsic::getDeclaration(Loc->getModule(),
+                                           Intrinsic::umul_with_overflow, Ty);
+    CallInst *Mul = Builder.CreateCall(MulF, {AbsStep, TruncTripCount}, "mul");
+    MulV = Builder.CreateExtractValue(Mul, 0, "mul.result");
+    OfMul = Builder.CreateExtractValue(Mul, 1, "mul.overflow");
+  }
 
   // Compute:
   //   Start + |Step| * Backedge < Start