[llvm] [InstCombine] Detect `(x ^ \-x)` as a ~Mask (PR #84868)

[via llvm-commits]

https://github.com/goldsteinn created https://github.com/llvm/llvm-project/pull/84868

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


>From 854777d3151804312265cf6d30fe04a592be169f Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n at gmail.com>
Date: Mon, 11 Mar 2024 22:52:20 -0500
Subject: [PATCH 1/2] [InstCombine] Add test for detecting `(x ^ -x)` as a
 ~Mask; NFC

---
 .../InstCombine/icmp-and-lowbit-mask.ll       | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll b/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
index 640a95b0561602..903d70685ab531 100644
--- a/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
@@ -453,6 +453,25 @@ 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:%.*]] = sub i8 0, [[Y:%.*]]
+; CHECK-NEXT:    [[NOTMASK0:%.*]] = xor i8 [[NEG_Y]], [[Y]]
+; CHECK-NEXT:    [[NOTMASK:%.*]] = select i1 [[COND:%.*]], i8 [[NOTMASK0]], i8 -8
+; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], [[NOTMASK]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[AND]], 0
+; 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

>From 5237ddabd1d56c942cb994275771d562d790385f Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n at gmail.com>
Date: Mon, 11 Mar 2024 22:52:29 -0500
Subject: [PATCH 2/2] [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.
---
 llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp  | 7 +++++--
 llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll | 7 +++----
 2 files changed, 8 insertions(+), 6 deletions(-)

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 903d70685ab531..07060922895846 100644
--- a/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-and-lowbit-mask.ll
@@ -456,11 +456,10 @@ define i1 @src_is_notmask_shl(i8 %x_in, i8 %y, i1 %cond) {
 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:%.*]] = sub i8 0, [[Y:%.*]]
+; CHECK-NEXT:    [[NEG_Y:%.*]] = add i8 [[Y:%.*]], -1
 ; CHECK-NEXT:    [[NOTMASK0:%.*]] = xor i8 [[NEG_Y]], [[Y]]
-; CHECK-NEXT:    [[NOTMASK:%.*]] = select i1 [[COND:%.*]], i8 [[NOTMASK0]], i8 -8
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], [[NOTMASK]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[AND]], 0
+; 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