[InstSimplify] Simplify both operands of select before comparing (#121753)

In the simplifySelectWithEquivalence fold, simplify both operands before
comparing them, instead of comparing one simplified operand with a
non-simplified operand. This is slightly more powerful.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4599,13 +4599,21 @@ static Value *simplifySelectWithEquivalence(Value *CmpLHS, Value *CmpRHS,
                                             Value *TrueVal, Value *FalseVal,
                                             const SimplifyQuery &Q,
                                             unsigned MaxRecurse) {
-  if (simplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, Q.getWithoutUndef(),
+  Value *SimplifiedFalseVal =
+      simplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, Q.getWithoutUndef(),
                              /* AllowRefinement */ false,
-                             /* DropFlags */ nullptr, MaxRecurse) == TrueVal)
-    return FalseVal;
-  if (simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, Q,
+                             /* DropFlags */ nullptr, MaxRecurse);
+  if (!SimplifiedFalseVal)
+    SimplifiedFalseVal = FalseVal;
+
+  Value *SimplifiedTrueVal =
+      simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, Q,
                              /* AllowRefinement */ true,
-                             /* DropFlags */ nullptr, MaxRecurse) == FalseVal)
+                             /* DropFlags */ nullptr, MaxRecurse);
+  if (!SimplifiedTrueVal)
+    SimplifiedTrueVal = TrueVal;
+
+  if (SimplifiedFalseVal == SimplifiedTrueVal)
     return FalseVal;
 
   return nullptr;

llvm/test/Transforms/InstCombine/select.ll

@@ -4453,11 +4453,8 @@ define i32 @src_no_trans_select_or_eq0_or_and(i32 %x, i32 %y) {
 
 define i32 @src_no_trans_select_or_eq0_or_xor(i32 %x, i32 %y) {
 ; CHECK-LABEL: @src_no_trans_select_or_eq0_or_xor(
-; CHECK-NEXT:    [[OR:%.*]] = or i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[OR0:%.*]] = icmp eq i32 [[OR]], 0
-; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[X]], [[Y]]
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[OR0]], i32 0, i32 [[XOR]]
-; CHECK-NEXT:    ret i32 [[COND]]
+; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i32 [[XOR]]
 ;
   %or = or i32 %x, %y
   %or0 = icmp eq i32 %or, 0
@@ -4492,11 +4489,8 @@ define i32 @src_no_trans_select_or_eq0_xor_or(i32 %x, i32 %y) {
 
 define i32 @src_no_trans_select_and_ne0_xor_or(i32 %x, i32 %y) {
 ; CHECK-LABEL: @src_no_trans_select_and_ne0_xor_or(
-; CHECK-NEXT:    [[OR:%.*]] = or i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[OR0_NOT:%.*]] = icmp eq i32 [[OR]], 0
-; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[X]], [[Y]]
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[OR0_NOT]], i32 0, i32 [[XOR]]
-; CHECK-NEXT:    ret i32 [[COND]]
+; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i32 [[XOR]]
 ;
   %or = or i32 %x, %y
   %or0 = icmp ne i32 %or, 0