[InstCombine] Simplify addends reordering logic

Previously some constants were not pushed to the top of the resulting
expression tree as intended by the algorithm. We can remove the logic
from simplifyFAdd and rely on SimplifyAssociativeOrCommutative to do
that.

Differential Revision: https://reviews.llvm.org/D117302

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -519,13 +519,6 @@ Value *FAddCombine::simplifyFAdd(AddendVect& Addends, unsigned InstrQuota) {
   unsigned NextTmpIdx = 0;
   FAddend TmpResult[3];
 
-  // Points to the constant addend of the resulting simplified expression.
-  // If the resulting expr has constant-addend, this constant-addend is
-  // desirable to reside at the top of the resulting expression tree. Placing
-  // constant close to supper-expr(s) will potentially reveal some optimization
-  // opportunities in super-expr(s).
-  const FAddend *ConstAdd = nullptr;
-
   // Simplified addends are placed <SimpVect>.
   AddendVect SimpVect;
 
@@ -541,6 +534,14 @@ Value *FAddCombine::simplifyFAdd(AddendVect& Addends, unsigned InstrQuota) {
     }
 
     Value *Val = ThisAddend->getSymVal();
+
+    // If the resulting expr has constant-addend, this constant-addend is
+    // desirable to reside at the top of the resulting expression tree. Placing
+    // constant close to super-expr(s) will potentially reveal some
+    // optimization opportunities in super-expr(s). Here we do not implement
+    // this logic intentionally and rely on SimplifyAssociativeOrCommutative
+    // call later.
+
     unsigned StartIdx = SimpVect.size();
     SimpVect.push_back(ThisAddend);
 
@@ -569,24 +570,15 @@ Value *FAddCombine::simplifyFAdd(AddendVect& Addends, unsigned InstrQuota) {
 
       // Pop all addends being folded and push the resulting folded addend.
       SimpVect.resize(StartIdx);
-      if (Val) {
-        if (!R.isZero()) {
-          SimpVect.push_back(&R);
-        }
-      } else {
-        // Don't push constant addend at this time. It will be the last element
-        // of <SimpVect>.
-        ConstAdd = &R;
+      if (!R.isZero()) {
+        SimpVect.push_back(&R);
       }
     }
   }
 
   assert((NextTmpIdx <= array_lengthof(TmpResult) + 1) &&
          "out-of-bound access");
 
-  if (ConstAdd)
-    SimpVect.push_back(ConstAdd);
-
   Value *Result;
   if (!SimpVect.empty())
     Result = createNaryFAdd(SimpVect, InstrQuota);

llvm/test/Transforms/Reassociate/fast-basictest.ll

@@ -353,8 +353,8 @@ define float @test12(float %X) {
 ; Check again with 'reassoc' and 'nsz' ('nsz' not technically required).
 define float @test12_reassoc_nsz(float %X) {
 ; CHECK-LABEL: @test12_reassoc_nsz(
-; CHECK-NEXT:    [[TMP1:%.*]] = fmul reassoc nsz float [[X:%.*]], 3.000000e+00
-; CHECK-NEXT:    [[TMP2:%.*]] = fsub reassoc nsz float 6.000000e+00, [[TMP1]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul reassoc nsz float [[X:%.*]], -3.000000e+00
+; CHECK-NEXT:    [[TMP2:%.*]] = fadd reassoc nsz float [[TMP1]], 6.000000e+00
 ; CHECK-NEXT:    ret float [[TMP2]]
 ;
   %A = fsub reassoc nsz float 1.000000e+00, %X