[PATCH] D63060: [InstCombine] usub.sat(a, b) + b => umax(a, b) (PR42178)

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, lebedev.ri, xbolva00.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

Fixes part of https://bugs.llvm.org/show_bug.cgi?id=42178. We'll also want to handle `uadd.sat(a, b) - b`, which is more complicated.

We will expand umax back to usubsat in the backend if profitable.


Repository:
  rL LLVM

https://reviews.llvm.org/D63060

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/saturating-add-sub.ll


Index: llvm/test/Transforms/InstCombine/saturating-add-sub.ll
===================================================================
--- llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -1044,9 +1044,9 @@
 
 define i8 @test_scalar_usub_add(i8 %a, i8 %b) {
 ; CHECK-LABEL: @test_scalar_usub_add(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[RES:%.*]] = add i8 [[SAT]], [[B]]
-; CHECK-NEXT:    ret i8 [[RES]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i8 [[B]], i8 [[A]]
+; CHECK-NEXT:    ret i8 [[TMP2]]
 ;
   %sat = call i8 @llvm.usub.sat.i8(i8 %a, i8 %b)
   %res = add i8 %sat, %b
@@ -1068,9 +1068,9 @@
 
 define i8 @test_scalar_usub_add_commuted(i8 %a, i8 %b) {
 ; CHECK-LABEL: @test_scalar_usub_add_commuted(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[RES:%.*]] = add i8 [[SAT]], [[B]]
-; CHECK-NEXT:    ret i8 [[RES]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i8 [[B]], i8 [[A]]
+; CHECK-NEXT:    ret i8 [[TMP2]]
 ;
   %sat = call i8 @llvm.usub.sat.i8(i8 %a, i8 %b)
   %res = add i8 %b, %sat
@@ -1090,9 +1090,9 @@
 
 define i8 @test_scalar_usub_add_const(i8 %a) {
 ; CHECK-LABEL: @test_scalar_usub_add_const(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 42)
-; CHECK-NEXT:    [[RES:%.*]] = add nuw i8 [[SAT]], 42
-; CHECK-NEXT:    ret i8 [[RES]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt i8 [[A:%.*]], 42
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i8 [[A]], i8 42
+; CHECK-NEXT:    ret i8 [[TMP2]]
 ;
   %sat = call i8 @llvm.usub.sat.i8(i8 %a, i8 42)
   %res = add i8 %sat, 42
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1302,6 +1302,14 @@
   if (Instruction *SatAdd = foldToUnsignedSaturatedAdd(I))
     return SatAdd;
 
+  // usub.sat(A, B) + A  => umax(A, B)
+  if (match(&I, m_c_BinOp(
+          m_OneUse(m_Intrinsic<Intrinsic::usub_sat>(m_Value(A), m_Value(B))),
+          m_Deferred(B)))) {
+    return replaceInstUsesWith(I,
+        Builder.CreateSelect(Builder.CreateICmpULT(A, B), B, A));
+  }
+
   return Changed ? &I : nullptr;
 }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D63060.203740.patch
Type: text/x-patch
Size: 2538 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190609/b6d03097/attachment.bin>