[InstCombine] Improve coverage of foldSelectValueEquivalence

1) We don't need the `noundef` check if the new simplification is a
constant.

2) There are a few more cases we can easily cover.
    - If f(Y) simplifies to Y
    - If replace X with Y makes X one-use (and make Y non one-use).

This cleans up regressions from folding multiuse:
    `(icmp eq/ne (sub/xor x, y), 0)` -> `(icmp eq/ne x, y)`.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1285,40 +1285,96 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
     Swapped = true;
   }
 
-  // In X == Y ? f(X) : Z, try to evaluate f(Y) and replace the operand.
-  // Make sure Y cannot be undef though, as we might pick different values for
-  // undef in the icmp and in f(Y). Additionally, take care to avoid replacing
-  // X == Y ? X : Z with X == Y ? Y : Z, as that would lead to an infinite
-  // replacement cycle.
   Value *CmpLHS = Cmp.getOperand(0), *CmpRHS = Cmp.getOperand(1);
-  if (TrueVal != CmpLHS &&
-      isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT)) {
-    if (Value *V = simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
-                                          /* AllowRefinement */ true))
-      // Require either the replacement or the simplification result to be a
-      // constant to avoid infinite loops.
-      // FIXME: Make this check more precise.
-      if (isa<Constant>(CmpRHS) || isa<Constant>(V))
-        return replaceOperand(Sel, Swapped ? 2 : 1, V);
+  auto ReplaceLHSOpWithRHSOp = [&](Value *OldOp,
+                                   Value *NewOp) -> Instruction * {
+    // In X == Y ? f(X) : Z, try to evaluate f(Y) and replace the operand.
+    // Take care to avoid replacing X == Y ? X : Z with X == Y ? Y : Z, as that
+    // would lead to an infinite replacement cycle.
+    // If we will be able to evaluate f(Y) to a constant, we can allow undef,
+    // otherwise Y cannot be undef as we might pick different values for undef
+    // in the icmp and in f(Y).
+    if (TrueVal == OldOp)
+      return nullptr;
+
+    std::optional<bool> IsNeverUndefCached;
+    auto IsNeverUndef = [&](Value *Op) {
+      if (!IsNeverUndefCached.has_value())
+        IsNeverUndefCached =
+            isGuaranteedNotToBeUndefOrPoison(Op, SQ.AC, &Sel, &DT);
+      return *IsNeverUndefCached;
+    };
 
+    if (Value *V = simplifyWithOpReplaced(TrueVal, OldOp, NewOp, SQ,
+                                          /* AllowRefinement */ true)) {
+      // Need some gurantees about the new simplified op to ensure we don't inf
+      // loop.
+      // If we simplify to a constant, replace.
+      bool ShouldReplace = match(V, m_ImmConstant());
+      bool NeedsNoUndef = !ShouldReplace;
+      // Or replace if either NewOp is a constant
+      if (!ShouldReplace && match(NewOp, m_ImmConstant()))
+        ShouldReplace = true;
+      // Or if we end up simplifying f(Y) -> Y i.e: Old & New -> New & New ->
+      // New.
+      if (!ShouldReplace && V == NewOp)
+        ShouldReplace = true;
+
+      // Finally, if we are going to create a new one-use instruction, replace.
+      if (!ShouldReplace && isa<Instruction>(OldOp) && OldOp->hasNUses(2) &&
+          (!isa<Instruction>(NewOp) || !NewOp->hasOneUse()))
+        ShouldReplace = true;
+
+      // Unless we simplify the new instruction to a constant, need to ensure Y
+      // is not undef.
+      if (NeedsNoUndef && ShouldReplace)
+        ShouldReplace = IsNeverUndef(NewOp);
+
+      if (ShouldReplace)
+        return replaceOperand(Sel, Swapped ? 2 : 1, V);
+    }
+    // If we can't simplify, but we will either:
+    //  1) Create a new binop where both ops are NewOp i.e (add x, y) is "worse"
+    //     than (add y, y) in this case, wait until the second call so we don't
+    //     miss a one-use simplification.
+    //  2) Create a new one-use instruction.
+    // proceed.
+    else if (match(TrueVal, m_c_BinOp(m_Specific(OldOp), m_Specific(NewOp))) ||
+             (isa<Instruction>(TrueVal) && TrueVal->hasOneUse() &&
+              isa<Instruction>(OldOp) && OldOp->hasNUses(2) &&
+              (!isa<Instruction>(NewOp) || !NewOp->hasOneUse()))) {
+      auto *TrueIns = cast<Instruction>(TrueVal);
+      for (unsigned OpIdx = 0; OpIdx < TrueIns->getNumOperands(); ++OpIdx) {
+        if (TrueIns->getOperand(OpIdx) == OldOp) {
+          // Need to ensure NewOp is noundef (same reason as above). Wait until
+          // the last moment to do this check as it can be relatively expensive.
+          if (!IsNeverUndef(NewOp))
+            break;
+          TrueIns->setOperand(OpIdx, NewOp);
+          return replaceOperand(Sel, Swapped ? 2 : 1, TrueIns);
+        }
+      }
+    }
     // Even if TrueVal does not simplify, we can directly replace a use of
     // CmpLHS with CmpRHS, as long as the instruction is not used anywhere
     // else and is safe to speculatively execute (we may end up executing it
     // with different operands, which should not cause side-effects or trigger
     // undefined behavior). Only do this if CmpRHS is a constant, as
     // profitability is not clear for other cases.
     // FIXME: Support vectors.
-    if (match(CmpRHS, m_ImmConstant()) && !match(CmpLHS, m_ImmConstant()) &&
-        !Cmp.getType()->isVectorTy())
-      if (replaceInInstruction(TrueVal, CmpLHS, CmpRHS))
+    if (OldOp == CmpLHS && match(NewOp, m_ImmConstant()) &&
+        !match(OldOp, m_ImmConstant()) && !Cmp.getType()->isVectorTy() &&
+        IsNeverUndef(NewOp))
+      if (replaceInInstruction(TrueVal, OldOp, NewOp))
         return &Sel;
-  }
-  if (TrueVal != CmpRHS &&
-      isGuaranteedNotToBeUndefOrPoison(CmpLHS, SQ.AC, &Sel, &DT))
-    if (Value *V = simplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, SQ,
-                                          /* AllowRefinement */ true))
-      if (isa<Constant>(CmpLHS) || isa<Constant>(V))
-        return replaceOperand(Sel, Swapped ? 2 : 1, V);
+
+    return nullptr;
+  };
+
+  if (Instruction *R = ReplaceLHSOpWithRHSOp(CmpLHS, CmpRHS))
+    return R;
+  if (Instruction *R = ReplaceLHSOpWithRHSOp(CmpRHS, CmpLHS))
+    return R;
 
   auto *FalseInst = dyn_cast<Instruction>(FalseVal);
   if (!FalseInst)

llvm/test/Transforms/InstCombine/abs-1.ll

@@ -852,11 +852,8 @@ define i8 @abs_diff_signed_sgt_nuw_extra_use3(i8 %a, i8 %b) {
 
 define i32 @abs_diff_signed_slt_swap_wrong_pred1(i32 %a, i32 %b) {
 ; CHECK-LABEL: @abs_diff_signed_slt_swap_wrong_pred1(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[SUB_BA:%.*]] = sub nsw i32 [[B]], [[A]]
-; CHECK-NEXT:    [[SUB_AB:%.*]] = sub nsw i32 [[A]], [[B]]
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[CMP]], i32 [[SUB_BA]], i32 [[SUB_AB]]
-; CHECK-NEXT:    ret i32 [[COND]]
+; CHECK-NEXT:    [[SUB_AB:%.*]] = sub nsw i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    ret i32 [[SUB_AB]]
 ;
   %cmp = icmp eq i32 %a, %b
   %sub_ba = sub nsw i32 %b, %a

llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll

@@ -6,8 +6,7 @@ declare void @use.i8(i8)
 define i8 @replace_with_y_noundef(i8 %x, i8 noundef %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_y_noundef(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[AND]], i8 [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Y]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %cmp = icmp eq i8 %x, %y
@@ -20,8 +19,7 @@ define i8 @replace_with_x_noundef(i8 noundef %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_x_noundef(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    call void @use.i1(i1 [[CMP]])
-; CHECK-NEXT:    [[AND:%.*]] = or i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Z:%.*]], i8 [[AND]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Z:%.*]], i8 [[X]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %cmp = icmp ne i8 %x, %y
@@ -50,7 +48,7 @@ define i8 @replace_with_y_for_new_oneuse(i8 noundef %xx, i8 noundef %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_y_for_new_oneuse(
 ; CHECK-NEXT:    [[X:%.*]] = mul i8 [[XX:%.*]], 13
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    [[ADD:%.*]] = add nuw i8 [[X]], [[Y]]
+; CHECK-NEXT:    [[ADD:%.*]] = shl nuw i8 [[Y]], 1
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[ADD]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
@@ -66,7 +64,7 @@ define i8 @replace_with_x_for_new_oneuse(i8 noundef %xx, i8 noundef %yy, i8 %z,
 ; CHECK-NEXT:    [[X:%.*]] = mul i8 [[XX:%.*]], 13
 ; CHECK-NEXT:    [[Y:%.*]] = add i8 [[YY:%.*]], [[W:%.*]]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[MUL:%.*]] = mul i8 [[X]], [[Y]]
+; CHECK-NEXT:    [[MUL:%.*]] = mul i8 [[X]], [[X]]
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[MUL]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
@@ -83,7 +81,7 @@ define i8 @replace_with_x_for_simple_binop(i8 noundef %xx, i8 %yy, i8 %z, i8 %w)
 ; CHECK-NEXT:    [[X:%.*]] = mul i8 [[XX:%.*]], 13
 ; CHECK-NEXT:    [[Y:%.*]] = add i8 [[YY:%.*]], [[W:%.*]]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[MUL:%.*]] = mul i8 [[X]], [[Y]]
+; CHECK-NEXT:    [[MUL:%.*]] = mul i8 [[X]], [[X]]
 ; CHECK-NEXT:    call void @use.i8(i8 [[Y]])
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[MUL]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
@@ -115,7 +113,7 @@ define i8 @replace_with_none_for_new_oneuse_fail_maybe_undef(i8 %xx, i8 %y, i8 %
 define i8 @replace_with_y_for_simple_binop(i8 %x, i8 noundef %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_y_for_simple_binop(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[MUL:%.*]] = mul nsw i8 [[X]], [[Y]]
+; CHECK-NEXT:    [[MUL:%.*]] = mul nsw i8 [[Y]], [[Y]]
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[MUL]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;

llvm/test/Transforms/InstCombine/select.ll

@@ -2787,8 +2787,7 @@ define <2 x i8> @select_replacement_add_eq_vec_nonuniform(<2 x i8> %x, <2 x i8>
 define <2 x i8> @select_replacement_add_eq_vec_poison(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @select_replacement_add_eq_vec_poison(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 1, i8 poison>
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i8> [[X]], <i8 1, i8 1>
-; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[CMP]], <2 x i8> [[ADD]], <2 x i8> [[Y:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[CMP]], <2 x i8> <i8 2, i8 poison>, <2 x i8> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i8> [[SEL]]
 ;
   %cmp = icmp eq <2 x i8> %x, <i8 1, i8 poison>
@@ -2839,8 +2838,7 @@ define i8 @select_replacement_sub_noundef(i8 %x, i8 noundef %y, i8 %z) {
 define i8 @select_replacement_sub(i8 %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @select_replacement_sub(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[SUB:%.*]] = sub i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[SUB]], i8 [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 0, i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %cmp = icmp eq i8 %x, %y