Another version of patch for D25987

[xbolva00]

Tests?

[kristina]

Please add code reviewers to your differential, a good place to start would be CODE_OWNERS.txt to see who is responsible for what, that will make sure you have at least one assigned reviewer for your code (since Herald hooks don't always fire especially when the title is not descriptive) and please try to be more descriptive in the differential message, when it's blank it's hard to relate it to others and D25987 should be mentioned in the the body for Phabricator to pick it up and link it to the previous differential.

[yulia_koval]

This is some old version of already commited patch, I think we need to close this.

lib/Transforms/InstCombine/InstCombineSelect.cpp

static Value *foldSelectICmpAndOr(const SelectInst &SI, Value *TrueVal,
   ICmpInst::Predicate Pred = IC->getPredicate();
   if ((Pred == ICmpInst::ICMP_NE && OrOnFalseVal) ||
       (Pred == ICmpInst::ICMP_EQ && OrOnTrueVal))
     V = Builder->CreateXor(V, *C2);
 
   return Builder->CreateOr(V, Y);
 }
 
+static Value *foldSelectICmpAndSub(SelectInst &SI, Value *TrueVal,
+                                   Value *FalseVal,
+                                   InstCombiner::BuilderTy *Builder) {
+  const ICmpInst *IC = dyn_cast<ICmpInst>(SI.getCondition());
+  Value *CmpLHS = IC->getOperand(0);
+  Value *CmpRHS = IC->getOperand(1);
+
+  if (match(FalseVal, m_Sub(m_Value(CmpLHS), m_Value(CmpRHS))) &&
+      match(TrueVal, m_Zero())) {
+    Value *A = Builder->CreateSelect(SI.getCondition(), CmpRHS, CmpLHS);
+    return Builder->CreateSub(A, CmpRHS);
+  }
+
+  return nullptr;
+}
+
 /// Attempt to fold a cttz/ctlz followed by a icmp plus select into a single
 /// call to cttz/ctlz with flag 'is_zero_undef' cleared.
 ///
 /// For example, we can fold the following code sequence:
 /// \code
 ///   %0 = tail call i32 @llvm.cttz.i32(i32 %x, i1 true)
 ///   %1 = icmp ne i32 %x, 0
 ///   %2 = select i1 %1, i32 %0, i32 32
if (IsBitTest) {
       if (V)
         return replaceInstUsesWith(SI, V);
     }
   }
 
   if (Value *V = foldSelectICmpAndOr(SI, TrueVal, FalseVal, Builder))
     return replaceInstUsesWith(SI, V);
 
+  if (Value *V = foldSelectICmpAndSub(SI, TrueVal, FalseVal, Builder))
+    return replaceInstUsesWith(SI, V);
+
   if (Value *V = foldSelectCttzCtlz(ICI, TrueVal, FalseVal, Builder))
     return replaceInstUsesWith(SI, V);
 
   return Changed ? &SI : nullptr;
 }
 
 
 /// SI is a select whose condition is a PHI node (but the two may be in