[llvm] r352536 - [InstCombine] canonicalize cmp/select form of uadd saturate with constant

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Tue Jan 29 12:02:45 2019
New Revision: 352536

URL: http://llvm.org/viewvc/llvm-project?rev=352536&view=rev
Log:
[InstCombine] canonicalize cmp/select form of uadd saturate with constant

I'm circling back around to a loose end from D51929.

The backend (either CGP or DAG) doesn't recognize this pattern, so we end up with different asm for these IR variants.

Regardless of any future changes to canonicalize to saturation/overflow intrinsics, we want to get raw IR variations 
into the minimal number of raw IR forms. If/when we can canonicalize to intrinsics, that will make that step easier.

  Pre: C2 == ~C1
  %a = add i32 %x, C1
  %c = icmp ugt i32 %x, C2
  %r = select i1 %c, i32 -1, i32 %a
  =>
  %a = add i32 %x, C1
  %c2 = icmp ult i32 %x, C2
  %r = select i1 %c2, i32 %a, i32 -1

  https://rise4fun.com/Alive/pkH

Differential Revision: https://reviews.llvm.org/D57352

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
    llvm/trunk/test/Transforms/InstCombine/saturating-add-sub.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp?rev=352536&r1=352535&r2=352536&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp Tue Jan 29 12:02:45 2019
@@ -675,6 +675,23 @@ static Value *canonicalizeSaturatedSubtr
   return IsNegative ? Builder.CreateSub(B, Max) : Builder.CreateSub(Max, B);
 }
 
+static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
+                                       InstCombiner::BuilderTy &Builder) {
+  // Match an unsigned saturated add with constant.
+  Value *X = Cmp->getOperand(0);
+  const APInt *CmpC, *AddC;
+  if (!Cmp->hasOneUse() || Cmp->getPredicate() != ICmpInst::ICMP_ULT ||
+      !match(Cmp->getOperand(1), m_APInt(CmpC)) || !match(FVal, m_AllOnes()) ||
+      !match(TVal, m_Add(m_Specific(X), m_APInt(AddC))) || ~(*AddC) != *CmpC)
+    return nullptr;
+
+  // Commute compare and select operands:
+  // select (icmp ult X, C), (add X, ~C), -1 -->
+  // select (icmp ugt X, C), -1, (add X, ~C)
+  Value *NewCmp = Builder.CreateICmp(ICmpInst::ICMP_UGT, X, Cmp->getOperand(1));
+  return Builder.CreateSelect(NewCmp, FVal, TVal);
+}
+
 /// 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.
 ///
@@ -1048,6 +1065,9 @@ Instruction *InstCombiner::foldSelectIns
   if (Value *V = canonicalizeSaturatedSubtract(ICI, TrueVal, FalseVal, Builder))
     return replaceInstUsesWith(SI, V);
 
+  if (Value *V = canonicalizeSaturatedAdd(ICI, TrueVal, FalseVal, Builder))
+    return replaceInstUsesWith(SI, V);
+
   return Changed ? &SI : nullptr;
 }
 

Modified: llvm/trunk/test/Transforms/InstCombine/saturating-add-sub.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/saturating-add-sub.ll?rev=352536&r1=352535&r2=352536&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/saturating-add-sub.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/saturating-add-sub.ll Tue Jan 29 12:02:45 2019
@@ -655,9 +655,9 @@ define i32 @uadd_sat_constant(i32 %x) {
 define i32 @uadd_sat_constant_commute(i32 %x) {
 ; CHECK-LABEL: @uadd_sat_constant_commute(
 ; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], 42
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[X]], -43
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[C]], i32 [[A]], i32 -1
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt i32 [[X]], -43
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i32 -1, i32 [[A]]
+; CHECK-NEXT:    ret i32 [[TMP2]]
 ;
   %a = add i32 %x, 42
   %c = icmp ult i32 %x, -43
@@ -681,9 +681,9 @@ define <4 x i32> @uadd_sat_constant_vec(
 define <4 x i32> @uadd_sat_constant_vec_commute(<4 x i32> %x) {
 ; CHECK-LABEL: @uadd_sat_constant_vec_commute(
 ; CHECK-NEXT:    [[A:%.*]] = add <4 x i32> [[X:%.*]], <i32 42, i32 42, i32 42, i32 42>
-; CHECK-NEXT:    [[C:%.*]] = icmp ult <4 x i32> [[X]], <i32 -43, i32 -43, i32 -43, i32 -43>
-; CHECK-NEXT:    [[R:%.*]] = select <4 x i1> [[C]], <4 x i32> [[A]], <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>
-; CHECK-NEXT:    ret <4 x i32> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt <4 x i32> [[X]], <i32 -43, i32 -43, i32 -43, i32 -43>
+; CHECK-NEXT:    [[TMP2:%.*]] = select <4 x i1> [[TMP1]], <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>, <4 x i32> [[A]]
+; CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 ;
   %a = add <4 x i32> %x, <i32 42, i32 42, i32 42, i32 42>
   %c = icmp ult <4 x i32> %x, <i32 -43, i32 -43, i32 -43, i32 -43>