[llvm] [InstCombine] Fold negation of unsigned div of non-negatives (PR #84951)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Antonio Frighetto (antoniofrighetto)

<details>
<summary>Changes</summary>

Let InstCombine carry out the following fold: `sub 0, (udiv nneg X, nneg C)` -> `sdiv nneg X, -C`.
Proofs: https://alive2.llvm.org/ce/z/9vyBvs.

Not sure if we need to go through `assumptionsFor` to update AC for the second assume.


---
Full diff: https://github.com/llvm/llvm-project/pull/84951.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp (+25-1) 
- (modified) llvm/test/Transforms/InstCombine/sub.ll (+63) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index aaf7184a5562cd..7ec55c6aef00d4 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2062,6 +2062,26 @@ static Instruction *foldSubOfMinMax(BinaryOperator &I,
   return nullptr;
 }
 
+/// Fold `sub 0, (udiv nneg X, nneg C)` into `sdiv nneg X, -C`
+static Instruction *foldNegationOfUDivOfNonNegatives(BinaryOperator &I,
+                                                     InstCombinerImpl &IC) {
+  Value *RHS = I.getOperand(1);
+  Value *X, *C;
+
+  if (match(RHS, m_OneUse(m_UDiv(m_Value(X), m_Value(C))))) {
+    KnownBits KnownX = IC.computeKnownBits(X, 0, &I);
+    KnownBits KnownC = IC.computeKnownBits(C, 0, &I);
+    if (KnownX.isNonNegative() && KnownC.isNonNegative()) {
+      bool HasNSW = I.hasNoSignedWrap();
+      auto *NegC = HasNSW ? IC.Builder.CreateNSWNeg(C, C->getName() + ".neg")
+                          : IC.Builder.CreateNeg(C, C->getName() + ".neg");
+      return BinaryOperator::CreateSDiv(X, NegC);
+    }
+  }
+
+  return nullptr;
+}
+
 Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
   if (Value *V = simplifySubInst(I.getOperand(0), I.getOperand(1),
                                  I.hasNoSignedWrap(), I.hasNoUnsignedWrap(),
@@ -2153,8 +2173,12 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
                                         Op1, *this))
       return BinaryOperator::CreateAdd(NegOp1, Op0);
   }
-  if (IsNegation)
+  if (IsNegation) {
+    if (Instruction *Res = foldNegationOfUDivOfNonNegatives(I, *this))
+      return Res;
+
     return TryToNarrowDeduceFlags(); // Should have been handled in Negator!
+  }
 
   // (A*B)-(A*C) -> A*(B-C) etc
   if (Value *V = foldUsingDistributiveLaws(I))
diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index 249b5673c8acfd..7cd1a0ea1646c7 100644
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -2626,3 +2626,66 @@ define i8 @sub_of_adds_2xc(i8 %x, i8 %y) {
   %r = sub i8 %xc, %yc
   ret i8 %r
 }
+
+define i8 @test_neg_of_udiv_of_nonnegs(i8 %a, i8 %b) {
+; CHECK-LABEL: @test_neg_of_udiv_of_nonnegs(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND1:%.*]] = icmp sgt i8 [[A:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND1]])
+; CHECK-NEXT:    [[COND2:%.*]] = icmp sgt i8 [[B:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND2]])
+; CHECK-NEXT:    ret i8 0
+;
+entry:
+  %cond1 = icmp sgt i8 %a, -1
+  call void @llvm.assume(i1 %cond1)
+  %cond2 = icmp sgt i8 %b, -1
+  call void @llvm.assume(i1 %cond2)
+  %div = udiv i8 %a, %b
+  %neg = sub nuw i8 0, %div
+  ret i8 %neg
+}
+
+define i8 @test_neg_of_udiv_of_nonnegs_2(i8 %a, i8 %b) {
+; CHECK-LABEL: @test_neg_of_udiv_of_nonnegs_2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND1:%.*]] = icmp sgt i8 [[A:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND1]])
+; CHECK-NEXT:    [[COND2:%.*]] = icmp sgt i8 [[B:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND2]])
+; CHECK-NEXT:    [[B_NEG:%.*]] = sub nsw i8 0, [[B]]
+; CHECK-NEXT:    [[NEG:%.*]] = sdiv i8 [[A]], [[B_NEG]]
+; CHECK-NEXT:    ret i8 [[NEG]]
+;
+entry:
+  %cond1 = icmp sgt i8 %a, -1
+  call void @llvm.assume(i1 %cond1)
+  %cond2 = icmp sgt i8 %b, -1
+  call void @llvm.assume(i1 %cond2)
+  %div = udiv i8 %a, %b
+  %neg = sub nsw i8 0, %div
+  ret i8 %neg
+}
+
+define i8 @test_neg_of_udiv_of_nonnegs_3(i8 %a, i8 %b) {
+; CHECK-LABEL: @test_neg_of_udiv_of_nonnegs_3(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND1:%.*]] = icmp sgt i8 [[A:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND1]])
+; CHECK-NEXT:    [[COND2:%.*]] = icmp sgt i8 [[B:%.*]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND2]])
+; CHECK-NEXT:    [[B_NEG:%.*]] = sub nsw i8 0, [[B]]
+; CHECK-NEXT:    [[NEG:%.*]] = sdiv i8 [[A]], [[B_NEG]]
+; CHECK-NEXT:    ret i8 [[NEG]]
+;
+entry:
+  %cond1 = icmp sgt i8 %a, -1
+  call void @llvm.assume(i1 %cond1)
+  %cond2 = icmp sgt i8 %b, -1
+  call void @llvm.assume(i1 %cond2)
+  %div = udiv i8 %a, %b
+  %neg = sub i8 0, %div
+  ret i8 %neg
+}
+
+declare void @llvm.assume(i1)

``````````

</details>


https://github.com/llvm/llvm-project/pull/84951