[llvm] InstCombine: improve optimizations for ceiling division with no overflow (PR #142869)
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Wed Jun 4 15:37:00 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: None (gaynor-anthropic)
<details>
<summary>Changes</summary>
fixes #<!-- -->142497
The contents of this pull request were substantially written using claude-code. I've reviewed to the best of my ability (it's been years since I did any compilers work).
---
Full diff: https://github.com/llvm/llvm-project/pull/142869.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp (+44)
- (modified) llvm/test/Transforms/InstCombine/add.ll (+157)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index a9ac5ff9b9c89..16ebd7bceff63 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1787,6 +1787,50 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
if (Instruction *Ashr = foldAddToAshr(I))
return Ashr;
+ // Ceiling division by power-of-2:
+ // (X >> log2(N)) + zext(X & (N-1) != 0) --> (X + (N-1)) >> log2(N)
+ // This is valid when adding (N-1) to X doesn't overflow.
+ {
+ Value *X = nullptr, *Cmp = nullptr;
+ const APInt *ShiftAmt = nullptr, *Mask = nullptr;
+ CmpPredicate Pred;
+
+ // Match: (X >> C) + zext((X & Mask) != 0)
+ // or: zext((X & Mask) != 0) + (X >> C)
+ Value *Op0 = I.getOperand(0);
+ Value *Op1 = I.getOperand(1);
+
+ // Try matching with shift on left, zext on right
+ bool Matched = false;
+ if (match(Op0, m_LShr(m_Value(X), m_APInt(ShiftAmt))) &&
+ match(Op1, m_ZExt(m_Value(Cmp)))) {
+ Matched = match(Cmp, m_ICmp(Pred, m_And(m_Specific(X), m_APInt(Mask)),
+ m_ZeroInt()));
+ } else if (match(Op1, m_LShr(m_Value(X), m_APInt(ShiftAmt))) &&
+ match(Op0, m_ZExt(m_Value(Cmp)))) {
+ Matched = match(Cmp, m_ICmp(Pred, m_And(m_Specific(X), m_APInt(Mask)),
+ m_ZeroInt()));
+ }
+
+ if (Matched &&
+ Pred == ICmpInst::ICMP_NE &&
+ ShiftAmt && ShiftAmt->uge(1) && ShiftAmt->ult(BitWidth) &&
+ Mask && *Mask == (APInt(BitWidth, 1) << *ShiftAmt) - 1) {
+
+ // Check if X + Mask doesn't overflow
+ Constant *MaskC = ConstantInt::get(X->getType(), *Mask);
+ bool WillNotOverflowUnsigned = willNotOverflowUnsignedAdd(X, MaskC, I);
+
+ if (WillNotOverflowUnsigned) {
+ // (X + Mask) >> ShiftAmt
+ bool WillNotOverflowSigned = willNotOverflowSignedAdd(X, MaskC, I);
+ Value *Add = Builder.CreateAdd(X, MaskC, "", WillNotOverflowUnsigned,
+ WillNotOverflowSigned);
+ return BinaryOperator::CreateLShr(Add, ConstantInt::get(X->getType(), *ShiftAmt));
+ }
+ }
+ }
+
// (~X) + (~Y) --> -2 - (X + Y)
{
// To ensure we can save instructions we need to ensure that we consume both
diff --git a/llvm/test/Transforms/InstCombine/add.ll b/llvm/test/Transforms/InstCombine/add.ll
index 495f99824652d..d364082eab317 100644
--- a/llvm/test/Transforms/InstCombine/add.ll
+++ b/llvm/test/Transforms/InstCombine/add.ll
@@ -4273,4 +4273,161 @@ define i32 @fold_zext_nneg_add_const_fail2(i8 %x) {
}
declare void @llvm.assume(i1)
+declare i32 @llvm.ctlz.i32(i32, i1)
+
+; Ceiling division by power-of-2: (x >> log2(N)) + ((x & (N-1)) != 0) -> (x + (N-1)) >> log2(N)
+; This is only valid when x + (N-1) doesn't overflow
+
+; Test with known range that prevents overflow
+define noundef range(i32 0, 100) i32 @ceil_div_by_8_known_range(i32 noundef range(i32 0, 100) %x) {
+; CHECK-LABEL: @ceil_div_by_8_known_range(
+; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i32 [[X:%.*]], 7
+; CHECK-NEXT: [[R:%.*]] = lshr i32 [[TMP1]], 3
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 3
+ %and = and i32 %x, 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add i32 %shr, %ext
+ ret i32 %r
+}
+
+; Test with the exact IR from the original testcase
+define noundef range(i32 0, 6) i32 @ceil_div_from_clz(i32 noundef %v) {
+; CHECK-LABEL: @ceil_div_from_clz(
+; CHECK-NEXT: [[CTLZ:%.*]] = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 [[V:%.*]], i1 false)
+; CHECK-NEXT: [[TMP1:%.*]] = sub nuw nsw i32 39, [[CTLZ]]
+; CHECK-NEXT: [[R:%.*]] = lshr i32 [[TMP1]], 3
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %ctlz = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 %v, i1 false)
+ %sub = sub nuw nsw i32 32, %ctlz
+ %shr = lshr i32 %sub, 3
+ %and = and i32 %sub, 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add nuw nsw i32 %shr, %ext
+ ret i32 %r
+}
+
+; Vector version with known range
+define <2 x i32> @ceil_div_by_8_vec_range(<2 x i32> range(i32 0, 1000) %x) {
+; CHECK-LABEL: @ceil_div_by_8_vec_range(
+; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw <2 x i32> [[X:%.*]], splat (i32 7)
+; CHECK-NEXT: [[R:%.*]] = lshr <2 x i32> [[TMP1]], splat (i32 3)
+; CHECK-NEXT: ret <2 x i32> [[R]]
+;
+ %shr = lshr <2 x i32> %x, <i32 3, i32 3>
+ %and = and <2 x i32> %x, <i32 7, i32 7>
+ %cmp = icmp ne <2 x i32> %and, <i32 0, i32 0>
+ %ext = zext <2 x i1> %cmp to <2 x i32>
+ %r = add <2 x i32> %shr, %ext
+ ret <2 x i32> %r
+}
+
+; Ceiling division by 16 with known range
+define i16 @ceil_div_by_16_i16(i16 range(i16 0, 1000) %x) {
+; CHECK-LABEL: @ceil_div_by_16_i16(
+; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i16 [[X:%.*]], 15
+; CHECK-NEXT: [[R:%.*]] = lshr i16 [[TMP1]], 4
+; CHECK-NEXT: ret i16 [[R]]
+;
+ %shr = lshr i16 %x, 4
+ %and = and i16 %x, 15
+ %cmp = icmp ne i16 %and, 0
+ %ext = zext i1 %cmp to i16
+ %r = add i16 %shr, %ext
+ ret i16 %r
+}
+
+; Negative test: no overflow guarantee - should NOT optimize
+define i32 @ceil_div_by_8_no_overflow_info(i32 %x) {
+; CHECK-LABEL: @ceil_div_by_8_no_overflow_info(
+; CHECK-NEXT: [[SHR:%.*]] = lshr i32 [[X:%.*]], 3
+; CHECK-NEXT: [[AND:%.*]] = and i32 [[X]], 7
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[AND]], 0
+; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT: [[R:%.*]] = add nuw nsw i32 [[SHR]], [[EXT]]
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 3
+ %and = and i32 %x, 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add i32 %shr, %ext
+ ret i32 %r
+}
+
+; Negative test: nuw on final add doesn't help
+define i32 @ceil_div_by_8_only_nuw_on_add(i32 %x) {
+; CHECK-LABEL: @ceil_div_by_8_only_nuw_on_add(
+; CHECK-NEXT: [[SHR:%.*]] = lshr i32 [[X:%.*]], 3
+; CHECK-NEXT: [[AND:%.*]] = and i32 [[X]], 7
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[AND]], 0
+; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT: [[R:%.*]] = add nuw nsw i32 [[SHR]], [[EXT]]
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 3
+ %and = and i32 %x, 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add nuw i32 %shr, %ext ; nuw here doesn't prove x+7 won't overflow
+ ret i32 %r
+}
+
+; Negative test: wrong mask
+define i32 @ceil_div_wrong_mask(i32 range(i32 0, 100) %x) {
+; CHECK-LABEL: @ceil_div_wrong_mask(
+; CHECK-NEXT: [[SHR:%.*]] = lshr i32 [[X:%.*]], 3
+; CHECK-NEXT: [[AND:%.*]] = and i32 [[X]], 6
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[AND]], 0
+; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT: [[R:%.*]] = add nuw nsw i32 [[SHR]], [[EXT]]
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 3
+ %and = and i32 %x, 6 ; Wrong mask: should be 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add i32 %shr, %ext
+ ret i32 %r
+}
+
+; Negative test: wrong shift amount
+define i32 @ceil_div_wrong_shift(i32 range(i32 0, 100) %x) {
+; CHECK-LABEL: @ceil_div_wrong_shift(
+; CHECK-NEXT: [[SHR:%.*]] = lshr i32 [[X:%.*]], 4
+; CHECK-NEXT: [[AND:%.*]] = and i32 [[X]], 7
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[AND]], 0
+; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT: [[R:%.*]] = add nuw nsw i32 [[SHR]], [[EXT]]
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 4 ; Shift by 4, but mask is 7 (should be 15)
+ %and = and i32 %x, 7
+ %cmp = icmp ne i32 %and, 0
+ %ext = zext i1 %cmp to i32
+ %r = add i32 %shr, %ext
+ ret i32 %r
+}
+
+; Negative test: wrong comparison
+define i32 @ceil_div_wrong_cmp(i32 range(i32 0, 100) %x) {
+; CHECK-LABEL: @ceil_div_wrong_cmp(
+; CHECK-NEXT: [[SHR:%.*]] = lshr i32 [[X:%.*]], 3
+; CHECK-NEXT: [[AND:%.*]] = and i32 [[X]], 7
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[AND]], 0
+; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT: [[R:%.*]] = add nuw nsw i32 [[SHR]], [[EXT]]
+; CHECK-NEXT: ret i32 [[R]]
+;
+ %shr = lshr i32 %x, 3
+ %and = and i32 %x, 7
+ %cmp = icmp eq i32 %and, 0 ; Wrong: should be ne
+ %ext = zext i1 %cmp to i32
+ %r = add i32 %shr, %ext
+ ret i32 %r
+}
declare void @fake_func(i32)
``````````
</details>
https://github.com/llvm/llvm-project/pull/142869
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