[DAG] SimplifyDemandedBits - ensure we drop NSW/NUW flags when we simplify a SHL node's input

We already do this for variable shifts, but we missed it for constant shifts

Fixes llvm#69965

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -1786,8 +1786,17 @@ bool TargetLowering::SimplifyDemandedBits(
 
       APInt InDemandedMask = DemandedBits.lshr(ShAmt);
       if (SimplifyDemandedBits(Op0, InDemandedMask, DemandedElts, Known, TLO,
-                               Depth + 1))
+                               Depth + 1)) {
+        SDNodeFlags Flags = Op.getNode()->getFlags();
+        if (Flags.hasNoSignedWrap() || Flags.hasNoUnsignedWrap()) {
+          // Disable the nsw and nuw flags. We can no longer guarantee that we
+          // won't wrap after simplification.
+          Flags.setNoSignedWrap(false);
+          Flags.setNoUnsignedWrap(false);
+          Op->setFlags(Flags);
+        }
         return true;
+      }
       assert(!Known.hasConflict() && "Bits known to be one AND zero?");
       Known.Zero <<= ShAmt;
       Known.One <<= ShAmt;

llvm/test/CodeGen/X86/pr69965.ll

@@ -10,10 +10,12 @@ define i64 @PR69965(ptr %input_ptrs, ptr %output_ptrs) {
 ; X86-NEXT:    movl (%eax), %eax
 ; X86-NEXT:    movzbl (%eax), %eax
 ; X86-NEXT:    notl %eax
-; X86-NEXT:    movzbl %al, %edx
-; X86-NEXT:    shll $8, %eax
+; X86-NEXT:    movl %eax, %edx
+; X86-NEXT:    shll $8, %edx
 ; X86-NEXT:    movl (%ecx), %ecx
-; X86-NEXT:    leal (%eax,%edx,2), %eax
+; X86-NEXT:    addb %al, %al
+; X86-NEXT:    movzbl %al, %eax
+; X86-NEXT:    orl %edx, %eax
 ; X86-NEXT:    orl $32768, %eax # imm = 0x8000
 ; X86-NEXT:    movw %ax, (%ecx)
 ; X86-NEXT:    xorl %eax, %eax
@@ -25,13 +27,14 @@ define i64 @PR69965(ptr %input_ptrs, ptr %output_ptrs) {
 ; X64-NEXT:    movq (%rdi), %rax
 ; X64-NEXT:    movzbl (%rax), %eax
 ; X64-NEXT:    notl %eax
-; X64-NEXT:    movzbl %al, %ecx
-; X64-NEXT:    # kill: def $eax killed $eax def $rax
+; X64-NEXT:    leal (%rax,%rax), %ecx
+; X64-NEXT:    # kill: def $eax killed $eax killed $rax
 ; X64-NEXT:    shll $8, %eax
 ; X64-NEXT:    movq (%rsi), %rdx
-; X64-NEXT:    leal (%rax,%rcx,2), %eax
-; X64-NEXT:    orl $32768, %eax # imm = 0x8000
-; X64-NEXT:    movw %ax, (%rdx)
+; X64-NEXT:    movzbl %cl, %ecx
+; X64-NEXT:    orl %eax, %ecx
+; X64-NEXT:    orl $32768, %ecx # imm = 0x8000
+; X64-NEXT:    movw %cx, (%rdx)
 ; X64-NEXT:    xorl %eax, %eax
 ; X64-NEXT:    retq
 entry: