[InstCombine] Detect (x ^ -x) as a ~Mask

[goldsteinn]

• [InstCombine] Add test for detecting (x ^ -x) as a ~Mask; NFC
• [InstCombine] Detect (x ^ -x) as a ~Mask

Proof: https://alive2.llvm.org/ce/z/TAFmPw

This is a lemma for clearing up some of the regressions that #84688
causes.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: None (goldsteinn)

Changes

• [InstCombine] Add test for detecting (x ^ -x) as a ~Mask; NFC
• [InstCombine] Detect (x ^ -x) as a ~Mask

---

Full diff: https://github.com/llvm/llvm-project/pull/84868.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+5-2)
• (modified) llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll (+18)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index e71f3e113b96e8..0dce0077bf1588 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4113,9 +4113,12 @@ static bool isMaskOrZero(const Value *V, bool Not, const SimplifyQuery &Q,
     if (match(V, m_Not(m_Value(X))))
       return isMaskOrZero(X, !Not, Q, Depth);
 
+    // (X ^ -X) is a ~Mask
+    if (Not)
+      return match(V, m_c_Xor(m_Value(X), m_Neg(m_Deferred(X))));
     // (X ^ (X - 1)) is a Mask
-    return !Not &&
-           match(V, m_c_Xor(m_Value(X), m_Add(m_Deferred(X), m_AllOnes())));
+    else
+      return match(V, m_c_Xor(m_Value(X), m_Add(m_Deferred(X), m_AllOnes())));
   case Instruction::Select:
     // c ? Mask0 : Mask1 is a Mask.
     return isMaskOrZero(I->getOperand(1), Not, Q, Depth) &&
diff --git a/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll b/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
index 640a95b0561602..07060922895846 100644
--- a/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
@@ -453,6 +453,24 @@ define i1 @src_is_notmask_shl(i8 %x_in, i8 %y, i1 %cond) {
   ret i1 %r
 }
 
+define i1 @src_is_notmask_x_xor_neg_x(i8 %x_in, i8 %y, i1 %cond) {
+; CHECK-LABEL: @src_is_notmask_x_xor_neg_x(
+; CHECK-NEXT:    [[X:%.*]] = xor i8 [[X_IN:%.*]], 123
+; CHECK-NEXT:    [[NEG_Y:%.*]] = add i8 [[Y:%.*]], -1
+; CHECK-NEXT:    [[NOTMASK0:%.*]] = xor i8 [[NEG_Y]], [[Y]]
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[COND:%.*]], i8 [[NOTMASK0]], i8 7
+; CHECK-NEXT:    [[R:%.*]] = icmp ule i8 [[X]], [[TMP3]]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+  %x = xor i8 %x_in, 123
+  %neg_y = sub i8 0, %y
+  %nmask0 = xor i8 %y, %neg_y
+  %notmask = select i1 %cond, i8 %nmask0, i8 -8
+  %and = and i8 %x, %notmask
+  %r = icmp eq i8 %and, 0
+  ret i1 %r
+}
+
 define i1 @src_is_notmask_shl_fail_multiuse_invert(i8 %x_in, i8 %y, i1 %cond) {
 ; CHECK-LABEL: @src_is_notmask_shl_fail_multiuse_invert(
 ; CHECK-NEXT:    [[X:%.*]] = xor i8 [[X_IN:%.*]], 122

[nikic]

LGTM