[InstCombine] Canonicalize clamp of float types to minmax in fast mode.

Summary:
This commit allows matchSelectPattern to recognize clamp of float
arguments in the presence of FMF the same way as already done for
integers.

This case is a little different though. With integers, given the
min/max pattern is recognized, DAGBuilder starts selecting MIN/MAX
"automatically". That is not the case for float, because for them only
full FMINNAN/FMINNUM/FMAXNAN/FMAXNUM ISD nodes exist and they do care
about NaNs. On the other hand, some backends (e.g. X86) have only
FMIN/FMAX nodes that do not care about NaNS and the former NAN/NUM
nodes are illegal thus selection is not happening. So I decided to do
such kind of transformation in IR (InstCombiner) instead of
complicating the logic in the backend.

Reviewers: spatel, jmolloy, majnemer, efriedma, craig.topper

Reviewed By: efriedma

Subscribers: hiraditya, javed.absar, n.bozhenov, llvm-commits

Patch by Andrei Elovikov <andrei.elovikov@intel.com>

Differential Revision: https://reviews.llvm.org/D33186

llvm-svn: 310054

1 files changed, 68 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 9ab98ce6175..34568713a05 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3994,6 +3994,62 @@ static bool isKnownNonZero(const Value *V) {
   return false;
 }
 
+/// Match clamp pattern for float types without care about NaNs or signed zeros.
+/// Given non-min/max outer cmp/select from the clamp pattern this
+/// function recognizes if it can be substitued by a "canonical" min/max
+/// pattern.
+static SelectPatternResult matchFastFloatClamp(CmpInst::Predicate Pred,
+                                               Value *CmpLHS, Value *CmpRHS,
+                                               Value *TrueVal, Value *FalseVal,
+                                               Value *&LHS, Value *&RHS) {
+  // Try to match
+  //   X < C1 ? C1 : Min(X, C2) --> Max(C1, Min(X, C2))
+  //   X > C1 ? C1 : Max(X, C2) --> Min(C1, Max(X, C2))
+  // and return description of the outer Max/Min.
+
+  // First, check if select has inverse order:
+  if (CmpRHS == FalseVal) {
+    std::swap(TrueVal, FalseVal);
+    Pred = CmpInst::getInversePredicate(Pred);
+  }
+
+  // Assume success now. If there's no match, callers should not use these anyway.
+  LHS = TrueVal;
+  RHS = FalseVal;
+
+  const APFloat *FC1;
+  if (CmpRHS != TrueVal || !match(CmpRHS, m_APFloat(FC1)) || !FC1->isFinite())
+    return {SPF_UNKNOWN, SPNB_NA, false};
+
+  const APFloat *FC2;
+  switch (Pred) {
+  case CmpInst::FCMP_OLT:
+  case CmpInst::FCMP_OLE:
+  case CmpInst::FCMP_ULT:
+  case CmpInst::FCMP_ULE:
+    if (match(FalseVal,
+              m_CombineOr(m_OrdFMin(m_Specific(CmpLHS), m_APFloat(FC2)),
+                          m_UnordFMin(m_Specific(CmpLHS), m_APFloat(FC2)))) &&
+        FC1->compare(*FC2) == APFloat::cmpResult::cmpLessThan)
+      return {SPF_FMAXNUM, SPNB_RETURNS_ANY, false};
+    break;
+  case CmpInst::FCMP_OGT:
+  case CmpInst::FCMP_OGE:
+  case CmpInst::FCMP_UGT:
+  case CmpInst::FCMP_UGE:
+    if (match(FalseVal,
+              m_CombineOr(m_OrdFMax(m_Specific(CmpLHS), m_APFloat(FC2)),
+                          m_UnordFMax(m_Specific(CmpLHS), m_APFloat(FC2)))) &&
+        FC1->compare(*FC2) == APFloat::cmpResult::cmpGreaterThan)
+      return {SPF_FMINNUM, SPNB_RETURNS_ANY, false};
+    break;
+  default:
+    break;
+  }
+
+  return {SPF_UNKNOWN, SPNB_NA, false};
+}
+
 /// Match non-obvious integer minimum and maximum sequences.
 static SelectPatternResult matchMinMax(CmpInst::Predicate Pred,
                                        Value *CmpLHS, Value *CmpRHS,
@@ -4201,7 +4257,18 @@ static SelectPatternResult matchSelectPattern(CmpInst::Predicate Pred,
     }
   }
 
-  return matchMinMax(Pred, CmpLHS, CmpRHS, TrueVal, FalseVal, LHS, RHS);
+  if (CmpInst::isIntPredicate(Pred))
+    return matchMinMax(Pred, CmpLHS, CmpRHS, TrueVal, FalseVal, LHS, RHS);
+
+  // According to (IEEE 754-2008 5.3.1), minNum(0.0, -0.0) and similar
+  // may return either -0.0 or 0.0, so fcmp/select pair has stricter
+  // semantics than minNum. Be conservative in such case.
+  if (NaNBehavior != SPNB_RETURNS_ANY ||
+      (!FMF.noSignedZeros() && !isKnownNonZero(CmpLHS) &&
+       !isKnownNonZero(CmpRHS)))
+    return {SPF_UNKNOWN, SPNB_NA, false};
+
+  return matchFastFloatClamp(Pred, CmpLHS, CmpRHS, TrueVal, FalseVal, LHS, RHS);
 }
 
 static Value *lookThroughCast(CmpInst *CmpI, Value *V1, Value *V2,