Backport PR27137: Fix matchSelectPattern for bitwise-not of unsigned compares

docs/ReleaseNotes.md

@@ -88,6 +88,8 @@ line upon naming the release. Refer to previous for appropriate section names.
   [#8526](https://github.com/microsoft/DirectXShaderCompiler/issues/8526).
 - Fixed incorrect shader results caused by stale overflow and exactness flags
   after bit-tracking dead code elimination.
+- Fixed an optimizer miscompile where selects of bitwise-not values guarded by
+  signed less-than or unsigned compares were treated as a signed minimum.
 
 ### Upcoming Preview Release
 

lib/Analysis/ValueTracking.cpp

@@ -3383,7 +3383,10 @@ static SelectPatternFlavor matchSelectPattern(ICmpInst::Predicate Pred,
 
     // Y >s C ? ~Y : ~C == ~Y <s ~C ? ~Y : ~C = SMIN(~Y, ~C)
     if (const auto *C2 = dyn_cast<ConstantInt>(FalseVal)) {
-      if (C1->getType() == C2->getType() && ~C1->getValue() == C2->getValue() &&
+      // HLSL Change - Add missing sgt check. Backport of rL264996 (PR27137):
+      // https://github.com/llvm/llvm-project/commit/56df0ec61033a5bb49b3620e70cc6a61e26b3e7a
+      if (Pred == ICmpInst::ICMP_SGT && C1->getType() == C2->getType() &&
+          ~C1->getValue() == C2->getValue() &&
           (match(TrueVal, m_Not(m_Specific(CmpLHS))) ||
            match(CmpLHS, m_Not(m_Specific(TrueVal))))) {
         LHS = TrueVal;

tools/clang/test/DXC/Passes/InstructionCombining/instcombine-select-of-not-minmax.ll

@@ -0,0 +1,28 @@
+; RUN: %dxopt %s -instcombine -S | FileCheck %s
+
+; Regression test for the backport of
+; https://github.com/llvm/llvm-project/commit/56df0ec61033a5bb49b3620e70cc6a61e26b3e7a
+; (rL264996), copied unchanged. matchSelectPattern matched (X pred C) ? ~X : ~C
+; as smin(~X, ~C) regardless of the predicate, but that only holds for sgt.
+; Before the fix, InstCombine folded @PR27137 to 'ret i32 -1', which is wrong
+; for negative %a.
+
+target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
+target triple = "dxil-ms-dx"
+
+define i32 @PR27137(i32 %a) {
+; CHECK-LABEL: @PR27137(
+; CHECK-NEXT:  %not_a = xor i32 %a, -1
+; CHECK-NEXT:  %c0 = icmp slt i32 %a, 0
+; CHECK-NEXT:  %s0 = select i1 %c0, i32 %not_a, i32 -1
+; CHECK-NEXT:  %c1 = icmp sgt i32 %s0, -1
+; CHECK-NEXT:  %s1 = select i1 %c1, i32 %s0, i32 -1
+; CHECK-NEXT:  ret i32 %s1
+
+  %not_a = xor i32 %a, -1
+  %c0 = icmp slt i32 %a, 0
+  %s0 = select i1 %c0, i32 %not_a, i32 -1
+  %c1 = icmp sgt i32 %s0, -1
+  %s1 = select i1 %c1, i32 %s0, i32 -1
+  ret i32 %s1
+}