[llvm] [llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq 0, (and X, V - 1)" (PR #152851)
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Sat Aug 9 05:09:51 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Pavel Skripkin (pskrgag)
<details>
<summary>Changes</summary>
This fold optimizes
```llvm
define i1 @<!-- -->src(i32 %num, i32 %val) {
%mask = add i32 %val, -1
%neg = sub nsw i32 0, %val
%num.biased = add i32 %num, %mask
%_2.sroa.0.0 = and i32 %num.biased, %neg
%_0 = icmp eq i32 %_2.sroa.0.0, %num
ret i1 %_0
}
```
to
```llvm
define i1 @<!-- -->tgt(i32 %num, i32 %val) {
%mask = add i32 %val, -1
%tmp = and i32 %num, %mask
%ret = icmp eq i32 %tmp, 0
ret i1 %ret
}
```
For power-of-two `val`.
Observed in real life for following code
```rust
pub fn is_aligned(num: usize) -> bool {
num.next_multiple_of(1 << 12) == num
}
```
which verifies that num is aligned to 4096.
Alive2 proof https://alive2.llvm.org/ce/z/QisECm
---
Full diff: https://github.com/llvm/llvm-project/pull/152851.diff
3 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+64)
- (modified) llvm/lib/Transforms/InstCombine/InstCombineInternal.h (+1)
- (modified) llvm/test/Transforms/InstCombine/icmp-add.ll (+90)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index cf94d28100488..722b03eb53f06 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1320,6 +1320,67 @@ Instruction *InstCombinerImpl::foldICmpWithZero(ICmpInst &Cmp) {
return nullptr;
}
+// Fold icmp eq (num + (val - 1)) & -val, num
+// to
+// icmp eq 0, (and num, val - 1)
+// For value being power of two
+Instruction *InstCombinerImpl::foldNextMultiply(ICmpInst &Cmp) {
+ Value *Op0 = Cmp.getOperand(0), *Op1 = Cmp.getOperand(1);
+ Value *Neg, *Add, *Num, *Mask, *Value;
+ CmpInst::Predicate Pred = Cmp.getPredicate();
+ const APInt *NegConst, *MaskConst, *NumCost;
+
+ if (Pred != ICmpInst::ICMP_EQ)
+ return nullptr;
+
+ // Match num + neg
+ if (!match(Op0, m_And(m_Value(Add), m_Value(Neg))))
+ return nullptr;
+
+ // Match num & mask
+ if (!match(Add, m_Add(m_Value(Num), m_Value(Mask))))
+ return nullptr;
+
+ // Check the constant case
+ if (match(Neg, m_APInt(NegConst)) && match(Mask, m_APInt(MaskConst))) {
+ // Mask + 1 should be a power-of-two
+ if (!(*MaskConst + 1).isPowerOf2())
+ return nullptr;
+
+ // Neg = -(Mask + 1)
+ if (*NegConst != -(*MaskConst + 1))
+ return nullptr;
+ } else {
+ // Match neg = sub 0, val
+ if (!match(Neg, m_Sub(m_Zero(), m_Value(Value))))
+ return nullptr;
+
+ // mask = %val - 1, which can be represented as sub %val, 1 or add %val, -1
+ if (!match(Mask, m_Add(m_Value(Value), m_AllOnes())) &&
+ !match(Mask, m_Sub(m_Value(Value), m_One())))
+ return nullptr;
+
+ // Value should be a known power-of-two.
+ if (!isKnownToBeAPowerOfTwo(Value, false, &Cmp))
+ return nullptr;
+ }
+
+ // Guard against weird special-case where Op1 gets optimized to constant. Leave it constant
+ // fonder.
+ if (match(Op1, m_APInt(NumCost)))
+ return nullptr;
+
+ if (!match(Op1, m_Value(Num)))
+ return nullptr;
+
+ // Create new icmp eq (num & (val - 1)), 0
+ auto NewAnd = Builder.CreateAnd(Num, Mask);
+ auto Zero = llvm::Constant::getNullValue(Num->getType());
+ auto ICmp = Builder.CreateICmp(CmpInst::ICMP_EQ, NewAnd, Zero);
+
+ return replaceInstUsesWith(Cmp, ICmp);
+}
+
/// Fold icmp Pred X, C.
/// TODO: This code structure does not make sense. The saturating add fold
/// should be moved to some other helper and extended as noted below (it is also
@@ -7644,6 +7705,9 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
if (Instruction *Res = foldICmpUsingKnownBits(I))
return Res;
+ if (Instruction *Res = foldNextMultiply(I))
+ return Res;
+
// Test if the ICmpInst instruction is used exclusively by a select as
// part of a minimum or maximum operation. If so, refrain from doing
// any other folding. This helps out other analyses which understand
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index c67e27e5b3e7c..5f83cb1b9ae28 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -721,6 +721,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
Instruction *foldICmpUsingKnownBits(ICmpInst &Cmp);
Instruction *foldICmpWithDominatingICmp(ICmpInst &Cmp);
Instruction *foldICmpWithConstant(ICmpInst &Cmp);
+ Instruction *foldNextMultiply(ICmpInst &Cmp);
Instruction *foldICmpUsingBoolRange(ICmpInst &I);
Instruction *foldICmpInstWithConstant(ICmpInst &Cmp);
Instruction *foldICmpInstWithConstantNotInt(ICmpInst &Cmp);
diff --git a/llvm/test/Transforms/InstCombine/icmp-add.ll b/llvm/test/Transforms/InstCombine/icmp-add.ll
index 1a41c1f3e1045..698619ab8aad1 100644
--- a/llvm/test/Transforms/InstCombine/icmp-add.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-add.ll
@@ -3300,3 +3300,93 @@ entry:
%cmp = icmp ult i32 %add, 253
ret i1 %cmp
}
+
+define i1 @val_is_aligend_sub(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_sub(
+; CHECK-NEXT: [[TMP1:%.*]] = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 [[NUM:%.*]])
+; CHECK-NEXT: [[POW:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT: call void @llvm.assume(i1 [[POW]])
+; CHECK-NEXT: [[NEG:%.*]] = add i32 [[NUM]], -1
+; CHECK-NEXT: [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED:%.*]], [[NEG]]
+; CHECK-NEXT: [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], 0
+; CHECK-NEXT: ret i1 [[_0]]
+;
+ %1 = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 %val)
+ %pow = icmp eq i32 %1, 1
+ call void @llvm.assume(i1 %pow)
+
+ %mask = sub i32 %val, 1
+ %neg = sub nsw i32 0, %val
+
+ %num.biased = add i32 %num, %mask
+ %_2.sroa.0.0 = and i32 %num.biased, %neg
+ %_0 = icmp eq i32 %_2.sroa.0.0, %num
+ ret i1 %_0
+}
+
+define i1 @val_is_aligend_add(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_add(
+; CHECK-NEXT: [[TMP1:%.*]] = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 [[NUM:%.*]])
+; CHECK-NEXT: [[POW:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT: call void @llvm.assume(i1 [[POW]])
+; CHECK-NEXT: [[NEG:%.*]] = add i32 [[NUM]], -1
+; CHECK-NEXT: [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED:%.*]], [[NEG]]
+; CHECK-NEXT: [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], 0
+; CHECK-NEXT: ret i1 [[_0]]
+;
+ %1 = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 %val)
+ %pow = icmp eq i32 %1, 1
+ call void @llvm.assume(i1 %pow)
+
+ %mask = add i32 %val, -1
+ %neg = sub nsw i32 0, %val
+
+ %num.biased = add i32 %num, %mask
+ %_2.sroa.0.0 = and i32 %num.biased, %neg
+ %_0 = icmp eq i32 %_2.sroa.0.0, %num
+ ret i1 %_0
+}
+
+define i1 @val_is_aligend_const_pow2(i32 %num) {
+; CHECK-LABEL: @val_is_aligend_const_pow2(
+; CHECK-NEXT: [[TMP1:%.*]] = and i32 [[NUM:%.*]], 4095
+; CHECK-NEXT: [[_0:%.*]] = icmp eq i32 [[TMP1]], 0
+; CHECK-NEXT: ret i1 [[_0]]
+;
+ %num.biased = add i32 %num, 4095
+ %_2.sroa.0.0 = and i32 %num.biased, -4096
+ %_0 = icmp eq i32 %_2.sroa.0.0, %num
+ ret i1 %_0
+}
+
+; Should not work for non-power-of-two cases
+define i1 @val_is_aligend_const_non-pow2(i32 %num) {
+; CHECK-LABEL: @val_is_aligend_const_non-pow2(
+; CHECK-NEXT: [[NUM_BIASED:%.*]] = add i32 [[NUM:%.*]], 6
+; CHECK-NEXT: [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED]], -7
+; CHECK-NEXT: [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], [[NUM]]
+; CHECK-NEXT: ret i1 [[_0]]
+;
+ %num.biased = add i32 %num, 6
+ %_2.sroa.0.0 = and i32 %num.biased, -7
+ %_0 = icmp eq i32 %_2.sroa.0.0, %num
+ ret i1 %_0
+}
+
+define i1 @val_is_aligend_non_pow(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_non_pow(
+; CHECK-NEXT: [[MASK:%.*]] = add i32 [[VAL:%.*]], -1
+; CHECK-NEXT: [[NEG:%.*]] = sub nsw i32 0, [[VAL]]
+; CHECK-NEXT: [[NUM_BIASED:%.*]] = add i32 [[NUM:%.*]], [[MASK]]
+; CHECK-NEXT: [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED]], [[NEG]]
+; CHECK-NEXT: [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], [[NUM]]
+; CHECK-NEXT: ret i1 [[_0]]
+;
+ %mask = add i32 %val, -1
+ %neg = sub nsw i32 0, %val
+
+ %num.biased = add i32 %num, %mask
+ %_2.sroa.0.0 = and i32 %num.biased, %neg
+ %_0 = icmp eq i32 %_2.sroa.0.0, %num
+ ret i1 %_0
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/152851
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