[llvm] [ConstraintElim] Decompose xor %a, -1 as -1 - %a in the signed system. (PR #213476)

[Florian Hahn via llvm-commits]

https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/213476

>From 9a2dbf5148072907003ce1a15fc1f3fcd119ead0 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Sat, 1 Aug 2026 19:24:48 +0100
Subject: [PATCH 1/2] [ConstraintElim] Precommit test

---
 .../ConstraintElimination/xor-as-sub.ll       | 46 +++++++++++++++++++
 1 file changed, 46 insertions(+)
 create mode 100644 llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll

diff --git a/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll b/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll
new file mode 100644
index 0000000000000..6a91d07d8027d
--- /dev/null
+++ b/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll
@@ -0,0 +1,46 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -passes=constraint-elimination -S %s | FileCheck %s
+
+define i1 @xor_neg_one_signed(i16 %count, i16 %i) {
+; CHECK-LABEL: define i1 @xor_neg_one_signed(
+; CHECK-SAME: i16 [[COUNT:%.*]], i16 [[I:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[PRE_0:%.*]] = icmp sgt i16 [[COUNT]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[PRE_0]])
+; CHECK-NEXT:    [[PRE_1:%.*]] = icmp sgt i16 [[I]], -1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[PRE_1]])
+; CHECK-NEXT:    [[PRE_2:%.*]] = icmp slt i16 [[I]], [[COUNT]]
+; CHECK-NEXT:    [[NEG:%.*]] = xor i16 [[I]], -1
+; CHECK-NEXT:    [[IDX:%.*]] = add nsw i16 [[COUNT]], [[NEG]]
+; CHECK-NEXT:    [[C:%.*]] = icmp slt i16 [[IDX]], [[COUNT]]
+; CHECK-NEXT:    ret i1 [[C]]
+;
+entry:
+  %pre.0 = icmp sgt i16 %count, -1
+  call void @llvm.assume(i1 %pre.0)
+  %pre.1 = icmp sgt i16 %i, -1
+  call void @llvm.assume(i1 %pre.1)
+  %pre.2 = icmp slt i16 %i, %count
+  %neg = xor i16 %i, -1
+  %idx = add nsw i16 %count, %neg
+  %c = icmp slt i16 %idx, %count
+  ret i1 %c
+}
+
+define i1 @xor_neg_1_unsigned(i16 %x) {
+; CHECK-LABEL: define i1 @xor_neg_1_unsigned(
+; CHECK-SAME: i16 [[X:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[PRE_0:%.*]] = icmp eq i16 [[X]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[PRE_0]])
+; CHECK-NEXT:    [[N:%.*]] = xor i16 [[X]], -1
+; CHECK-NEXT:    [[C:%.*]] = icmp ult i16 [[N]], 5
+; CHECK-NEXT:    ret i1 [[C]]
+;
+entry:
+  %pre.0  = icmp eq i16 %x, 0
+  call void @llvm.assume(i1 %pre.0)
+  %n = xor i16 %x, -1
+  %c = icmp ult i16 %n, 5
+  ret i1 %c
+}

>From 297f2354fc661827377b9dad475cefb5db0c3b55 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Thu, 30 Jul 2026 15:46:23 +0100
Subject: [PATCH 2/2] [ConstraintElim] Decompose `xor %a, -1` as `-1 - %a` in
 the signed system.

InstCombine canonicalizes sub nsw i8 -1, %a -> xor i8 %a, -1. Decompose
the XOR as `sub nsw -1, %a` in the signed system

Alive2 Proof: https://alive2.llvm.org/ce/z/f_YVjH

This triggers quite rarely in C/C++ workloads (no end-to-end changes in
https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/840), found
one instance in Blender. One simple end-to-end C example is
https://clang.godbolt.org/z/G343shYje

This is part of an effort to improve ConstraintElimination support for
IR generated by the Swift compiler, where such patterns are more common
due to a number of signed runtime checks.
---
 llvm/lib/Transforms/Scalar/ConstraintElimination.cpp     | 8 ++++++++
 llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll | 3 +--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 8b7018f6c8433..3d5ef33318c50 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -552,6 +552,14 @@ static Decomposition decompose(Value *V, const ConstraintInfo &Info,
       return V;
     }
 
+    // `xor %x, -1` is equivalent to `sub nsw -1, %x`.
+    if (match(V, m_Not(m_Value(Op0)))) {
+      Decomposition Result(-1);
+      if (!Result.sub(decompose(Op0, Info, IsSigned, DL)))
+        return Result;
+      return V;
+    }
+
     if (match(V, m_NSWSub(m_Value(Op0), m_Value(Op1)))) {
       auto ResA = decompose(Op0, Info, IsSigned, DL);
       auto ResB = decompose(Op1, Info, IsSigned, DL);
diff --git a/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll b/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll
index 6a91d07d8027d..d20cf3df23572 100644
--- a/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll
+++ b/llvm/test/Transforms/ConstraintElimination/xor-as-sub.ll
@@ -12,8 +12,7 @@ define i1 @xor_neg_one_signed(i16 %count, i16 %i) {
 ; CHECK-NEXT:    [[PRE_2:%.*]] = icmp slt i16 [[I]], [[COUNT]]
 ; CHECK-NEXT:    [[NEG:%.*]] = xor i16 [[I]], -1
 ; CHECK-NEXT:    [[IDX:%.*]] = add nsw i16 [[COUNT]], [[NEG]]
-; CHECK-NEXT:    [[C:%.*]] = icmp slt i16 [[IDX]], [[COUNT]]
-; CHECK-NEXT:    ret i1 [[C]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %pre.0 = icmp sgt i16 %count, -1