SROA generate a noundef instead of splatting a noundef int

[saethlin]

Fixes #145121

This is just a fix for the SROA behavior, and I think that will also avoid what EarlyCSE is doing. Which is a bit unfortunate, because I'd like to be well-motivated to fix both passes.

Where should I add a test for this? I think I should add a regression test, but I don't understand how the tests are organized.

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[nikic]

The SROA tests can be found here: https://github.com/llvm/llvm-project/tree/main/llvm/test/Transforms/SROA You can either create a new file or try to find an existing one for this code path -- the easiest way to do that is to add an assert and see which tests fail :)

It's worth noting that we have another copy of the memset splat generation code here, using a different implementation:

llvm-project/llvm/lib/Transforms/Utils/VNCoercion.cpp

Lines 415 to 442 in 4373463

	// We know that this method is only called when the mem transfer fully
	// provides the bits for the load.
	if (MemSetInst *MSI = dyn_cast<MemSetInst>(SrcInst)) {
	// memset(P, 'x', 1234) -> splat('x'), even if x is a variable, and
	// independently of what the offset is.
	Value *Val = MSI->getValue();
	if (LoadSize != 1)
	Val =
	Builder.CreateZExtOrBitCast(Val, IntegerType::get(Ctx, LoadSize * 8));
	Value *OneElt = Val;
	// Splat the value out to the right number of bits.
	for (unsigned NumBytesSet = 1; NumBytesSet != LoadSize;) {
	// If we can double the number of bytes set, do it.
	if (NumBytesSet * 2 <= LoadSize) {
	Value *ShVal = Builder.CreateShl(
	Val, ConstantInt::get(Val->getType(), NumBytesSet * 8));
	Val = Builder.CreateOr(Val, ShVal);
	NumBytesSet <<= 1;
	continue;
	}
	// Otherwise insert one byte at a time.
	Value *ShVal =
	Builder.CreateShl(Val, ConstantInt::get(Val->getType(), 1 * 8));
	Val = Builder.CreateOr(OneElt, ShVal);
	++NumBytesSet;
	}

We should probably unify these as a followup.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp -- llvm/lib/Transforms/Scalar/SROA.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Scalar/SROA.cpp b/llvm/lib/Transforms/Scalar/SROA.cpp
index 028bcde5a..403baed93 100644
--- a/llvm/lib/Transforms/Scalar/SROA.cpp
+++ b/llvm/lib/Transforms/Scalar/SROA.cpp
@@ -3222,14 +3222,14 @@ private:
 
     Type *SplatIntTy = Type::getIntNTy(VTy->getContext(), Size * 8);
     if (isa<UndefValue>(V)) {
-        V = UndefValue::get(VTy);
+      V = UndefValue::get(VTy);
     } else {
-        V = IRB.CreateMul(
-            IRB.CreateZExt(V, SplatIntTy, "zext"),
-            IRB.CreateUDiv(Constant::getAllOnesValue(SplatIntTy),
-                           IRB.CreateZExt(Constant::getAllOnesValue(V->getType()),
-                                          SplatIntTy)),
-            "isplat");
+      V = IRB.CreateMul(
+          IRB.CreateZExt(V, SplatIntTy, "zext"),
+          IRB.CreateUDiv(Constant::getAllOnesValue(SplatIntTy),
+                         IRB.CreateZExt(Constant::getAllOnesValue(V->getType()),
+                                        SplatIntTy)),
+          "isplat");
     }
     return V;
   }

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/Transforms/Scalar/SROA.cpp

The following files introduce new uses of undef:

• llvm/lib/Transforms/Scalar/SROA.cpp

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.