[PATCH] D69514: [InstCombine] Expand usub_sat patterns to handle constants

[Dave Green via Phabricator via llvm-commits]

dmgreen created this revision.
dmgreen added reviewers: nikic, lebedev.ri, spatel.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

The constants come through as add %x, -C, not a sub as would be expected. They need some extra matchers to canonicalise them towards usub_sat.

  ----------------------------------------
  Optimization: Forwards
  Precondition: true
    %cmp = icmp ugt i32 %a, 9
    %sub = add i32 %a, -10
    %sel = select i1 %cmp, i32 %sub, i32 0
  =>
    %sel = usub_sat %a, 10
  
  Done: 1
  Optimization is correct!
  
  ----------------------------------------
  Optimization: Backwards
  Precondition: true
    %cmp = icmp ult i32 %a, 2
    %sub = add i32 %a, -2
    %sel = select i1 %cmp, i32 %sub, i32 0
  =>
    %x = usub_sat 2, %a
    %sel = sub 0, %x
  
  Done: 1
  Optimization is correct!


https://reviews.llvm.org/D69514

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll


Index: llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
===================================================================
--- llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
+++ llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
@@ -160,10 +160,8 @@
 
 define i32 @max_sub_ugt_c1(i32 %a) {
 ; CHECK-LABEL: @max_sub_ugt_c1(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i32 [[A:%.*]], 1
-; CHECK-NEXT:    [[SUB:%.*]] = add i32 [[A]], -1
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 0
-; CHECK-NEXT:    ret i32 [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[A:%.*]], i32 1)
+; CHECK-NEXT:    ret i32 [[TMP1]]
 ;
   %cmp = icmp ugt i32 %a, 1
   %sub = add i32 %a, -1
@@ -173,10 +171,8 @@
 
 define i32 @max_sub_ugt_c10(i32 %a) {
 ; CHECK-LABEL: @max_sub_ugt_c10(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i32 [[A:%.*]], 10
-; CHECK-NEXT:    [[SUB:%.*]] = add i32 [[A]], -10
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 0
-; CHECK-NEXT:    ret i32 [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[A:%.*]], i32 10)
+; CHECK-NEXT:    ret i32 [[TMP1]]
 ;
   %cmp = icmp ugt i32 %a, 10
   %sub = add i32 %a, -10
@@ -211,10 +207,9 @@
 
 define i32 @max_sub_ult_c2(i32 %a) {
 ; CHECK-LABEL: @max_sub_ult_c2(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[A:%.*]], 2
-; CHECK-NEXT:    [[SUB:%.*]] = add i32 [[A]], -2
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 0
-; CHECK-NEXT:    ret i32 [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.usub.sat.i32(i32 2, i32 [[A:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = sub nsw i32 0, [[TMP1]]
+; CHECK-NEXT:    ret i32 [[TMP2]]
 ;
   %cmp = icmp ult i32 %a, 2
   %sub = add i32 %a, -2
Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -705,10 +705,16 @@
          "Unexpected isUnsigned predicate!");
 
   // Account for swapped form of subtraction: ((a > b) ? b - a : 0).
+  // Checking for both a-b and a+(-b) as a constant.
   bool IsNegative = false;
-  if (match(TrueVal, m_Sub(m_Specific(B), m_Specific(A))))
+  const APInt *C;
+  if (match(TrueVal, m_Sub(m_Specific(B), m_Specific(A))) ||
+      (match(A, m_APInt(C)) &&
+          match(TrueVal, m_Add(m_Specific(B), m_SpecificInt(-*C)))))
     IsNegative = true;
-  else if (!match(TrueVal, m_Sub(m_Specific(A), m_Specific(B))))
+  else if (!match(TrueVal, m_Sub(m_Specific(A), m_Specific(B))) &&
+           !(match(B, m_APInt(C)) &&
+             match(TrueVal, m_Add(m_Specific(A), m_SpecificInt(-*C)))))
     return nullptr;
 
   // If sub is used anywhere else, we wouldn't be able to eliminate it


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