[InstCombine] PR37603: low bit mask canonicalization
Summary:
This is [[ https://bugs.llvm.org/show_bug.cgi?id=37603 | PR37603 ]].
https://godbolt.org/g/VCMNpS
https://rise4fun.com/Alive/idM
When doing bit manipulations, it is quite common to calculate some bit mask,
and apply it to some value via `and`.
The typical C code looks like:
```
int mask_signed_add(int nbits) {
return (1 << nbits) - 1;
}
```
which is translated into (with `-O3`)
```
define dso_local i32 @mask_signed_add(int)(i32) local_unnamed_addr #0 {
%2 = shl i32 1, %0
%3 = add nsw i32 %2, -1
ret i32 %3
}
```
But there is a second, less readable variant:
```
int mask_signed_xor(int nbits) {
return ~(-(1 << nbits));
}
```
which is translated into (with `-O3`)
```
define dso_local i32 @mask_signed_xor(int)(i32) local_unnamed_addr #0 {
%2 = shl i32 -1, %0
%3 = xor i32 %2, -1
ret i32 %3
}
```
Since we created such a mask, it is quite likely that we will use it in `and` next.
And then we may get rid of `not` op by folding into `andn`.
But now that i have actually looked:
https://godbolt.org/g/VTUDmU
_some_ backend changes will be needed too.
We clearly loose `bzhi` recognition.
Reviewers: spatel, craig.topper, RKSimon
Reviewed By: spatel
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D47428
llvm-svn: 334127
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 6fd4aff..0ad0771 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1096,6 +1096,30 @@
return nullptr;
}
+/// Fold
+/// (1 << NBits) - 1
+/// Into:
+/// ~(-(1 << NBits))
+/// Because a 'not' is better for bit-tracking analysis and other transforms
+/// than an 'add'. The new shl is always nsw, and is nuw if old `and` was.
+static Instruction *canonicalizeLowbitMask(BinaryOperator &I,
+ InstCombiner::BuilderTy &Builder) {
+ Value *NBits;
+ if (!match(&I, m_Add(m_OneUse(m_Shl(m_One(), m_Value(NBits))), m_AllOnes())))
+ return nullptr;
+
+ Constant *MinusOne = Constant::getAllOnesValue(NBits->getType());
+ Value *NotMask = Builder.CreateShl(MinusOne, NBits, "notmask");
+ // Be wary of constant folding.
+ if (auto *BOp = dyn_cast<BinaryOperator>(NotMask)) {
+ // Always NSW. But NUW propagates from `add`.
+ BOp->setHasNoSignedWrap();
+ BOp->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
+ }
+
+ return BinaryOperator::CreateNot(NotMask, I.getName());
+}
+
Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
bool Changed = SimplifyAssociativeOrCommutative(I);
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
@@ -1347,6 +1371,9 @@
I.setHasNoUnsignedWrap(true);
}
+ if (Instruction *V = canonicalizeLowbitMask(I, Builder))
+ return V;
+
return Changed ? &I : nullptr;
}