[InstCombine] Simplifiy sdiv -X, X into X == INT_MIN ? 1 : -1

[Z572]

Alive2: https://alive2.llvm.org/ce/z/dHddwH
Fixes #69574

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-transforms

Author: Z572 (Z572)

Changes

Fixes #69574

---

Full diff: https://github.com/llvm/llvm-project/pull/71768.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (+14)
• (modified) llvm/test/Transforms/InstCombine/div.ll (+30)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index db0804380855e3a..a4460f0b3020918 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1544,6 +1544,20 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
     }
   }
 
+  // -(X * Y) / (X * Y) --> -1
+  if ((match(Op0, m_Neg(m_c_Mul(m_Value(X), m_Value(Y)))) &&
+       match(Op1, m_c_Mul(m_Specific(X), m_Specific(Y)))) ||
+      (match(Op0, m_c_Mul(m_Value(X), m_Value(Y))) &&
+       match(Op1, m_Neg(m_c_Mul(m_Specific(X), m_Specific(Y)))))) {
+    return BinaryOperator::CreateNeg(ConstantInt::get(Ty, 1));
+  };
+
+  // (Y - X) / (X - Y) --> -1
+  if (match(Op0, m_Sub(m_Value(Y), m_Value(X))) &&
+      match(Op1, m_Sub(m_Specific(X), m_Specific(Y)))) {
+    return BinaryOperator::CreateNeg(ConstantInt::get(Ty, 1));
+  }
+
   return nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/div.ll b/llvm/test/Transforms/InstCombine/div.ll
index 1f0081befb07dba..099c12d60f248df 100644
--- a/llvm/test/Transforms/InstCombine/div.ll
+++ b/llvm/test/Transforms/InstCombine/div.ll
@@ -1432,6 +1432,36 @@ define <2 x i8> @sdiv_sdiv_mul_nsw(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
   ret <2 x i8> %r
 }
 
+; -(X * Y) / (X * Y) --> -1
+define i32 @sdiv_neg_mul_mul(i32 %0, i32 %1) {
+; CHECK-LABEL: @sdiv_neg_mul_mul(
+; CHECK-NEXT:    ret i32 -1
+  %3 = mul i32 %1, %0
+  %4 = sub i32 0, %3
+  %5 = sdiv i32 %4, %3
+  ret i32 %5
+}
+
+; (X * Y) / -(X * Y) --> -1
+define i32 @sdiv_mul_neg_mul(i32 %0, i32 %1) {
+; CHECK-LABEL: @sdiv_mul_neg_mul(
+; CHECK-NEXT:    ret i32 -1
+  %3 = mul i32 %1, %0
+  %4 = sub i32 0, %3
+  %5 = sdiv i32 %3, %4
+  ret i32 %5
+}
+
+; (Y - X) / (X - Y) --> -1
+define i32 @sdiv_sub_sub(i32 %0, i32 %1) {
+; CHECK-LABEL: @sdiv_sub_sub(
+; CHECK-NEXT:    ret i32 -1
+  %3 = sub i32 %1, %0
+  %4 = sub nsw i32 %0, %1
+  %5 = sdiv i32 %3, %4
+  ret i32 %5
+}
+
 ; exact propagates
 
 define i8 @sdiv_sdiv_mul_nsw_exact_exact(i8 %x, i8 %y, i8 %z) {

[dtcxzyw]

Please attach alive2 proof.
This transform is incorrect when both the divisor and dividend are equal to INT_MIN.
Alive2: https://alive2.llvm.org/ce/z/VBRrGU

[dtcxzyw]

Alive2 proof: https://alive2.llvm.org/ce/z/AYRxRU

[github-actions]

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

[nikic]

As far as I can tell, all the tests you have added already fold without the patch: https://llvm.godbolt.org/z/z4W9KhP3b

Please adjust the test coverage to check cases that are actually newly supported with this patch.

[nikic]

I think the transform you actually want to implement is this one: https://alive2.llvm.org/ce/z/PBjgSM

The cases from #69574 lack the nsw flag, and this is the variant of the transform that doesn't need nsw.

[nikic]

Please also add a multi-use test (extra use of %neg). As a div is removed, we want to allow the transform with multi-use.

Please also test the case (x - y) / (y - x), which is one of the special cases isKnownNegation() handles.

[nikic]

LGTM apart from the final test nit.