[llvm] [AggressiveInstCombine] Guard memset with length in [0, 1] (PR #213240)

[Nikita Popov via llvm-commits]

================
@@ -2462,6 +2464,41 @@ static bool foldMulHigh(Instruction &I) {
   return false;
 }
 
+/// Guard a memset whose nonconstant length is known to be in [0, 1].
+/// Inserts a conditional branch around the memset and specialises the
+/// executed path to a one-byte store.
+static bool foldMemSetZeroOrOneLength(Instruction &I, const DataLayout &DL,
+                                      TargetLibraryInfo &TLI,
+                                      AssumptionCache &AC, DominatorTree &DT,
+                                      bool &MadeCFGChange) {
+  auto *MI = dyn_cast<MemSetInst>(&I);
+  if (!MI || isa<ConstantInt>(MI->getLength()))
+    return false;
+
+  SimplifyQuery SQ(DL, &TLI, &DT, &AC, nullptr, /*UseInstrInfo=*/true);
+  KnownBits KnownLen =
+      computeKnownBits(MI->getLength(), SQ.getWithInstruction(MI));
----------------
nikic wrote:

```suggestion
  SimplifyQuery SQ(DL, &TLI, &DT, &AC, MI);
  KnownBits KnownLen = computeKnownBits(MI->getLength());
```

https://github.com/llvm/llvm-project/pull/213240