[SDAG] Fix incorrect use of undef for boolean contents (PR63055)

FoldSetCC() returns UNDEF in a number of cases. However, the SetCC
result must follow BooleanContents. Unless the type is a
pre-legalization i1 or we have UndefinedBooleanContents, the use of
UNDEF will not uphold the requirement that the top bits are either
zero or match the low bit. In such cases, return zero instead.

Fixes #63055.

Differential Revision: https://reviews.llvm.org/D151883

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2436,6 +2436,16 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
                                 ISD::CondCode Cond, const SDLoc &dl) {
   EVT OpVT = N1.getValueType();
 
+  auto GetUndefBooleanConstant = [&]() {
+    if (VT.getScalarType() == MVT::i1 ||
+        TLI->getBooleanContents(OpVT) ==
+            TargetLowering::UndefinedBooleanContent)
+      return getUNDEF(VT);
+    // ZeroOrOne / ZeroOrNegative require specific values for the high bits,
+    // so we cannot use getUNDEF(). Return zero instead.
+    return getConstant(0, dl, VT);
+  };
+
   // These setcc operations always fold.
   switch (Cond) {
   default: break;
@@ -2465,12 +2475,12 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
     // icmp eq/ne X, undef -> undef.
     if ((N1.isUndef() || N2.isUndef()) &&
         (Cond == ISD::SETEQ || Cond == ISD::SETNE))
-      return getUNDEF(VT);
+      return GetUndefBooleanConstant();
 
     // If both operands are undef, we can return undef for int comparison.
     // icmp undef, undef -> undef.
     if (N1.isUndef() && N2.isUndef())
-      return getUNDEF(VT);
+      return GetUndefBooleanConstant();
 
     // icmp X, X -> true/false
     // icmp X, undef -> true/false because undef could be X.
@@ -2496,34 +2506,34 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
     switch (Cond) {
     default: break;
     case ISD::SETEQ:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETOEQ: return getBoolConstant(R==APFloat::cmpEqual, dl, VT,
                                              OpVT);
     case ISD::SETNE:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETONE: return getBoolConstant(R==APFloat::cmpGreaterThan ||
                                              R==APFloat::cmpLessThan, dl, VT,
                                              OpVT);
     case ISD::SETLT:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETOLT: return getBoolConstant(R==APFloat::cmpLessThan, dl, VT,
                                              OpVT);
     case ISD::SETGT:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETOGT: return getBoolConstant(R==APFloat::cmpGreaterThan, dl,
                                              VT, OpVT);
     case ISD::SETLE:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETOLE: return getBoolConstant(R==APFloat::cmpLessThan ||
                                              R==APFloat::cmpEqual, dl, VT,
                                              OpVT);
     case ISD::SETGE:  if (R==APFloat::cmpUnordered)
-                        return getUNDEF(VT);
+                        return GetUndefBooleanConstant();
                       [[fallthrough]];
     case ISD::SETOGE: return getBoolConstant(R==APFloat::cmpGreaterThan ||
                                          R==APFloat::cmpEqual, dl, VT, OpVT);
@@ -2568,7 +2578,7 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
     case 1: // Known true.
       return getBoolConstant(true, dl, VT, OpVT);
     case 2: // Undefined.
-      return getUNDEF(VT);
+      return GetUndefBooleanConstant();
     }
   }
 

llvm/test/CodeGen/X86/avx512-insert-extract.ll

@@ -1878,10 +1878,11 @@ define i96 @test_insertelement_variable_v96i1(<96 x i8> %a, i8 %b, i32 %index) n
 ; KNL-NEXT:    vpinsrb $14, 720(%rbp), %xmm3, %xmm3
 ; KNL-NEXT:    vpinsrb $15, 728(%rbp), %xmm3, %xmm3
 ; KNL-NEXT:    vinserti128 $1, %xmm3, %ymm2, %ymm2
-; KNL-NEXT:    vpcmpeqb %ymm0, %ymm2, %ymm0
-; KNL-NEXT:    vpternlogq $15, %zmm0, %zmm0, %zmm0
+; KNL-NEXT:    vpcmpeqb %ymm0, %ymm2, %ymm2
+; KNL-NEXT:    vpternlogq $15, %zmm2, %zmm2, %zmm2
 ; KNL-NEXT:    cmpb $0, 736(%rbp)
 ; KNL-NEXT:    vmovdqa %ymm0, {{[0-9]+}}(%rsp)
+; KNL-NEXT:    vmovdqa %ymm2, {{[0-9]+}}(%rsp)
 ; KNL-NEXT:    vmovdqa64 %zmm1, (%rsp)
 ; KNL-NEXT:    setne (%rsp,%rax)
 ; KNL-NEXT:    vpmovsxbd (%rsp), %zmm0

llvm/test/CodeGen/X86/setcc.ll

@@ -339,17 +339,16 @@ define i32 @PR55138(i32 %x) {
   ret i32 %and
 }
 
-; FIXME: Miscompile.
 define i64 @pr63055(double %arg) {
 ; X86-LABEL: pr63055:
 ; X86:       ## %bb.0:
-; X86-NEXT:    movl $255, %eax
+; X86-NEXT:    movl $1, %eax
 ; X86-NEXT:    xorl %edx, %edx
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: pr63055:
 ; X64:       ## %bb.0:
-; X64-NEXT:    movl $255, %eax
+; X64-NEXT:    movl $1, %eax
 ; X64-NEXT:    retq
   %fcmp = fcmp une double 0x7FF8000000000000, %arg
   %ext = zext i1 %fcmp to i64