[InstCombine] Canonicalize SPF_ABS to abs intrinc

Enable canonicalization of SPF_ABS and SPF_NABS to the abs intrinsic.

To be conservative, the one-use check on the comparison is retained,
this may be relaxed if all goes well.

It's pretty likely that this will uncover places that missing
handling for the abs() intrinsic. Please report any seen performance
regressions.

Differential Revision: https://reviews.llvm.org/D87188

Author: nikic

clang/test/CodeGen/builtins-wasm.c

@@ -328,26 +328,20 @@ u8x16 sub_saturate_u_i8x16(u8x16 x, u8x16 y) {
 
 i8x16 abs_i8x16(i8x16 v) {
   return __builtin_wasm_abs_i8x16(v);
-  // WEBASSEMBLY: %neg = sub <16 x i8> zeroinitializer, %v
-  // WEBASSEMBLY: %abscond = icmp slt <16 x i8> %v, zeroinitializer
-  // WEBASSEMBLY: %abs = select <16 x i1> %abscond, <16 x i8> %neg, <16 x i8> %v
-  // WEBASSEMBLY: ret <16 x i8> %abs
+  // WEBASSEMBLY: call <16 x i8> @llvm.abs.v16i8(<16 x i8> %v, i1 false)
+  // WEBASSEMBLY-NEXT: ret
 }
 
 i16x8 abs_i16x8(i16x8 v) {
   return __builtin_wasm_abs_i16x8(v);
-  // WEBASSEMBLY: %neg = sub <8 x i16> zeroinitializer, %v
-  // WEBASSEMBLY: %abscond = icmp slt <8 x i16> %v, zeroinitializer
-  // WEBASSEMBLY: %abs = select <8 x i1> %abscond, <8 x i16> %neg, <8 x i16> %v
-  // WEBASSEMBLY: ret <8 x i16> %abs
+  // WEBASSEMBLY: call <8 x i16> @llvm.abs.v8i16(<8 x i16> %v, i1 false)
+  // WEBASSEMBLY-NEXT: ret
 }
 
 i32x4 abs_i32x4(i32x4 v) {
   return __builtin_wasm_abs_i32x4(v);
-  // WEBASSEMBLY: %neg = sub <4 x i32> zeroinitializer, %v
-  // WEBASSEMBLY: %abscond = icmp slt <4 x i32> %v, zeroinitializer
-  // WEBASSEMBLY: %abs = select <4 x i1> %abscond, <4 x i32> %neg, <4 x i32> %v
-  // WEBASSEMBLY: ret <4 x i32> %abs
+  // WEBASSEMBLY: call <4 x i32> @llvm.abs.v4i32(<4 x i32> %v, i1 false)
+  // WEBASSEMBLY-NEXT: ret
 }
 
 i8x16 min_s_i8x16(i8x16 x, i8x16 y) {

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1064,89 +1064,29 @@ static Instruction *canonicalizeMinMaxWithConstant(SelectInst &Sel,
   return &Sel;
 }
 
-/// There are many select variants for each of ABS/NABS.
-/// In matchSelectPattern(), there are different compare constants, compare
-/// predicates/operands and select operands.
-/// In isKnownNegation(), there are different formats of negated operands.
-/// Canonicalize all these variants to 1 pattern.
-/// This makes CSE more likely.
+/// Canonicalize select-based abs/nabs to llvm.abs() intrinsic.
 static Instruction *canonicalizeAbsNabs(SelectInst &Sel, ICmpInst &Cmp,
                                         InstCombinerImpl &IC) {
   if (!Cmp.hasOneUse() || !isa<Constant>(Cmp.getOperand(1)))
     return nullptr;
 
-  // Choose a sign-bit check for the compare (likely simpler for codegen).
-  // ABS:  (X <s 0) ? -X : X
-  // NABS: (X <s 0) ? X : -X
   Value *LHS, *RHS;
   SelectPatternFlavor SPF = matchSelectPattern(&Sel, LHS, RHS).Flavor;
   if (SPF != SelectPatternFlavor::SPF_ABS &&
       SPF != SelectPatternFlavor::SPF_NABS)
     return nullptr;
 
-  Value *TVal = Sel.getTrueValue();
-  Value *FVal = Sel.getFalseValue();
-  assert(isKnownNegation(TVal, FVal) &&
-         "Unexpected result from matchSelectPattern");
-
-  // The compare may use the negated abs()/nabs() operand, or it may use
-  // negation in non-canonical form such as: sub A, B.
-  bool CmpUsesNegatedOp = match(Cmp.getOperand(0), m_Neg(m_Specific(TVal))) ||
-                          match(Cmp.getOperand(0), m_Neg(m_Specific(FVal)));
-
-  bool CmpCanonicalized = !CmpUsesNegatedOp &&
-                          match(Cmp.getOperand(1), m_ZeroInt()) &&
-                          Cmp.getPredicate() == ICmpInst::ICMP_SLT;
-  bool RHSCanonicalized = match(RHS, m_Neg(m_Specific(LHS)));
-
-  // Is this already canonical?
-  if (CmpCanonicalized && RHSCanonicalized)
-    return nullptr;
-
-  // If RHS is not canonical but is used by other instructions, don't
-  // canonicalize it and potentially increase the instruction count.
-  if (!RHSCanonicalized)
-    if (!(RHS->hasOneUse() || (RHS->hasNUses(2) && CmpUsesNegatedOp)))
-      return nullptr;
+  bool IntMinIsPoison = match(RHS, m_NSWNeg(m_Specific(LHS)));
+  Constant *IntMinIsPoisonC =
+      ConstantInt::get(Type::getInt1Ty(Sel.getContext()), IntMinIsPoison);
+  Instruction *Abs =
+      IC.Builder.CreateBinaryIntrinsic(Intrinsic::abs, LHS, IntMinIsPoisonC);
 
-  // Create the canonical compare: icmp slt LHS 0.
-  if (!CmpCanonicalized) {
-    Cmp.setPredicate(ICmpInst::ICMP_SLT);
-    Cmp.setOperand(1, ConstantInt::getNullValue(Cmp.getOperand(0)->getType()));
-    if (CmpUsesNegatedOp)
-      Cmp.setOperand(0, LHS);
-  }
-
-  // Create the canonical RHS: RHS = sub (0, LHS).
-  if (!RHSCanonicalized) {
-    assert(RHS->hasOneUse() && "RHS use number is not right");
-    RHS = IC.Builder.CreateNeg(LHS);
-    if (TVal == LHS) {
-      // Replace false value.
-      IC.replaceOperand(Sel, 2, RHS);
-      FVal = RHS;
-    } else {
-      // Replace true value.
-      IC.replaceOperand(Sel, 1, RHS);
-      TVal = RHS;
-    }
-  }
+  if (SPF == SelectPatternFlavor::SPF_NABS)
+    return IntMinIsPoison ? BinaryOperator::CreateNSWNeg(Abs)
+                          : BinaryOperator::CreateNeg(Abs);
 
-  // If the select operands do not change, we're done.
-  if (SPF == SelectPatternFlavor::SPF_NABS) {
-    if (TVal == LHS)
-      return &Sel;
-    assert(FVal == LHS && "Unexpected results from matchSelectPattern");
-  } else {
-    if (FVal == LHS)
-      return &Sel;
-    assert(TVal == LHS && "Unexpected results from matchSelectPattern");
-  }
-
-  // We are swapping the select operands, so swap the metadata too.
-  Sel.swapValues();
-  Sel.swapProfMetadata();
-  return &Sel;
+  return IC.replaceInstUsesWith(Sel, Abs);
 }
 
 /// If we have a select with an equality comparison, then we know the value in