[llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq (and X, V - 1), 0" #152851

pskrgag · 2025-08-09T12:09:12Z

This fold optimizes

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

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

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

… val - 1)"

llvmbot · 2025-08-09T12:09:46Z

@llvm/pr-subscribers-llvm-transforms

Author: Pavel Skripkin (pskrgag)

Changes

This fold optimizes

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

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

pub fn is_aligned(num: usize) -&gt; bool {
    num.next_multiple_of(1 &lt;&lt; 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 --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
+}

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

pskrgag · 2025-08-09T19:16:26Z

@dtcxzyw Big thanks you for review! Addressed your comments

github-actions · 2025-08-09T19:17:06Z

✅ With the latest revision this PR passed the C/C++ code formatter.

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

pskrgag · 2025-08-11T18:05:59Z

CI looks unrelated

  ERROR: test_modulelist_deadlock (TestStatusline.TestStatusline.test_modulelist_deadlock)
     Regression test for a deadlock that occurs when the status line is enabled before connecting

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

llvm/test/Transforms/InstCombine/icmp-add.ll

dtcxzyw

LGTM w/ comments resolved.

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

pskrgag · 2025-08-13T20:05:19Z

CI complains about CodeGen/Thumb2/mve-vcvt-fixed-to-float.ll:479. IR looks unrelated to my change

define arm_aapcs_vfpcc <8 x half> @vcvt_i16_15(<8 x i16> %0) {
; CHECK-LABEL: vcvt_i16_15:
; CHECK:       @ %bb.0:
; CHECK-NEXT:    vmov.i16 q1, #0x200
; CHECK-NEXT:    vcvt.f16.s16 q0, q0
; CHECK-NEXT:    vmul.f16 q0, q0, q1
; CHECK-NEXT:    bx lr
  %2 = sitofp <8 x i16> %0 to <8 x half>
  %3 = fmul ninf <8 x half> %2, <half 0xH0200, half 0xH0200, half 0xH0200, half 0xH0200, half 0xH0200, half 0xH0200, half 0xH0200, half 0xH0200>
  ret <8 x half> %3
}

pskrgag · 2025-08-14T06:30:51Z

Should i wait for CI to become green, or I can merge now?

nikic · 2025-08-14T07:21:45Z

You can go ahead and merge.

fold "icmp eq (num + (val - 1)) & -val, num" to "icmp eq 0, (and num,…

d8b5d15

… val - 1)"

pskrgag requested a review from nikic as a code owner August 9, 2025 12:09

pskrgag requested review from dtcxzyw and removed request for nikic August 9, 2025 12:09

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Aug 9, 2025

coding style...

93fd0c6

This was referenced Aug 9, 2025

Fuzz PR152851 dtcxzyw/llvm-mutation-based-fuzz-service#76

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Fuzz PR152851 dtcxzyw/llvm-fuzz-service#113

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dtcxzyw requested a review from nikic August 9, 2025 18:23

address review comments

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coding style....

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Task submission dtcxzyw/llvm-opt-benchmark#1312

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pre-commit: PR152851 dtcxzyw/llvm-opt-benchmark#2656

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review

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pskrgag added 2 commits August 10, 2025 21:12

learning llvm...

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use m_Defereded instead of m_Specific

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Fuzz PR152851 dtcxzyw/llvm-fuzz-service#114

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pre-commit: PR152851 dtcxzyw/llvm-opt-benchmark#2657

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don't open code bitwise not

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drop non-constant path as it happens to be non-profitable

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dtcxzyw changed the title ~~[llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq 0, (and X, V - 1)"~~ [llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq (and X, V - 1), 0" Aug 13, 2025

pskrgag added 3 commits August 13, 2025 21:38

style fixes

ee4c1a2

Merge branch 'main' into foldIcmp

98e655c

swap operands in comment

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comment fixes

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dtcxzyw mentioned this pull request Aug 14, 2025

[InstCombine] Propagate invariant.load metadata across unpacked loads #152186

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pskrgag merged commit 3014422 into llvm:main Aug 14, 2025
7 of 9 checks passed

[llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq (and X, V - 1), 0" #152851

[llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq (and X, V - 1), 0" #152851

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