[InstCombine] choose 1 form of abs and nabs as canonical

We already do this for min/max (see the blob above the diff), 
so we should do the same for abs/nabs.
A sign-bit check (<s 0) is used as a predicate for other IR 
transforms and it's likely the best for codegen.

This might solve the motivating cases for D47037 and D47041, 
but I think those patches still make sense. We can't guarantee 
this canonicalization if the icmp has more than one use.

Differential Revision: https://reviews.llvm.org/D47076

llvm-svn: 332819

1 files changed, 51 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index b7735c43dd7..9fe3c59e5e0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -794,6 +794,54 @@ canonicalizeMinMaxWithConstant(SelectInst &Sel, ICmpInst &Cmp,
   return &Sel;
 }
 
+/// There are 4 select variants for each of ABS/NABS (different compare
+/// constants, compare predicates, select operands). Canonicalize to 1 pattern.
+/// This makes CSE more likely.
+static Instruction *canonicalizeAbsNabs(SelectInst &Sel, ICmpInst &Cmp,
+                                        InstCombiner::BuilderTy &Builder) {
+  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;
+
+  // Is this already canonical?
+  if (match(Cmp.getOperand(1), m_ZeroInt()) &&
+      Cmp.getPredicate() == ICmpInst::ICMP_SLT)
+    return nullptr;
+
+  // Create the canonical compare.
+  Cmp.setPredicate(ICmpInst::ICMP_SLT);
+  Cmp.setOperand(1, ConstantInt::getNullValue(LHS->getType()));
+
+  // If the select operands do not change, we're done.
+  Value *TVal = Sel.getTrueValue();
+  Value *FVal = Sel.getFalseValue();
+  if (SPF == SelectPatternFlavor::SPF_NABS) {
+    if (TVal == LHS && match(FVal, m_Neg(m_Specific(TVal))))
+      return &Sel;
+    assert(FVal == LHS && match(TVal, m_Neg(m_Specific(FVal))) &&
+           "Unexpected results from matchSelectPattern");
+  } else {
+    if (FVal == LHS && match(TVal, m_Neg(m_Specific(FVal))))
+      return &Sel;
+    assert(TVal == LHS && match(FVal, m_Neg(m_Specific(TVal))) &&
+           "Unexpected results from matchSelectPattern");
+  }
+
+  // We are swapping the select operands, so swap the metadata too.
+  Sel.setTrueValue(FVal);
+  Sel.setFalseValue(TVal);
+  Sel.swapProfMetadata();
+  return &Sel;
+}
+
 /// Visit a SelectInst that has an ICmpInst as its first operand.
 Instruction *InstCombiner::foldSelectInstWithICmp(SelectInst &SI,
                                                   ICmpInst *ICI) {
@@ -803,6 +851,9 @@ Instruction *InstCombiner::foldSelectInstWithICmp(SelectInst &SI,
   if (Instruction *NewSel = canonicalizeMinMaxWithConstant(SI, *ICI, Builder))
     return NewSel;
 
+  if (Instruction *NewAbs = canonicalizeAbsNabs(SI, *ICI, Builder))
+    return NewAbs;
+
   bool Changed = adjustMinMax(SI, *ICI);
 
   if (Value *V = foldSelectICmpAnd(SI, ICI, Builder))