[PATCH] D156499: [InstCombine] Fold abs of known sign operand when source is sub

[Allen zhong via Phabricator via llvm-commits]

Allen created this revision.
Allen added reviewers: bcl5980, peterwaller-arm, RKSimon, goldstein.w.n, nikic.
Herald added subscribers: StephenFan, hiraditya.
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abs(x-y) --> x-y where x >= y
abs(x-y) --> y-x where x <= y


https://reviews.llvm.org/D156499

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/abs-intrinsic.ll


Index: llvm/test/Transforms/InstCombine/abs-intrinsic.ll
===================================================================
--- llvm/test/Transforms/InstCombine/abs-intrinsic.ll
+++ llvm/test/Transforms/InstCombine/abs-intrinsic.ll
@@ -452,6 +452,33 @@
   ret i32 %r
 }
 
+; https://alive2.llvm.org/ce/z/VSumU5
+define i32 @sub_abs_sge(i32 %x, i32 %y) {
+; CHECK-LABEL: @sub_abs_sge(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp slt i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[COND_END:%.*]], label [[COND_TRUE:%.*]]
+; CHECK:       cond.true:
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 [[X]], [[Y]]
+; CHECK-NEXT:    br label [[COND_END]]
+; CHECK:       cond.end:
+; CHECK-NEXT:    [[R:%.*]] = phi i32 [ [[SUB]], [[COND_TRUE]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+entry:
+  %cmp = icmp sge i32 %x, %y
+  br i1 %cmp, label %cond.true, label %cond.end
+
+cond.true:
+  %sub = sub nsw i32 %x, %y
+  %0 = call i32 @llvm.abs.i32(i32 %sub, i1 true)
+  br label %cond.end
+
+cond.end:
+  %r = phi i32 [ %0, %cond.true ], [ 0, %entry ]
+  ret i32 %r
+}
+
 define i32 @sub_abs_lt(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_abs_lt(
 ; CHECK-NEXT:  entry:
@@ -478,6 +505,33 @@
   ret i32 %r
 }
 
+; https://alive2.llvm.org/ce/z/9wQo6G
+define i32 @sub_abs_sle(i32 %x, i32 %y) {
+; CHECK-LABEL: @sub_abs_sle(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp sgt i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[COND_END:%.*]], label [[COND_TRUE:%.*]]
+; CHECK:       cond.true:
+; CHECK-NEXT:    [[SUB_NEG:%.*]] = sub i32 [[Y]], [[X]]
+; CHECK-NEXT:    br label [[COND_END]]
+; CHECK:       cond.end:
+; CHECK-NEXT:    [[R:%.*]] = phi i32 [ [[SUB_NEG]], [[COND_TRUE]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+entry:
+  %cmp = icmp sle i32 %x, %y
+  br i1 %cmp, label %cond.true, label %cond.end
+
+cond.true:
+  %sub = sub nsw i32 %x, %y
+  %0 = call i32 @llvm.abs.i32(i32 %sub, i1 true)
+  br label %cond.end
+
+cond.end:
+  %r = phi i32 [ %0, %cond.true ], [ 0, %entry ]
+  ret i32 %r
+}
+
 define i32 @sub_abs_lt_min_not_poison(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_abs_lt_min_not_poison(
 ; CHECK-NEXT:  entry:
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1024,7 +1024,13 @@
 
   Value *X, *Y;
   if (match(Op, m_NSWSub(m_Value(X), m_Value(Y))))
-    return isImpliedByDomCondition(ICmpInst::ICMP_SLT, X, Y, CxtI, DL);
+    if (std::optional<bool> Known =
+            isImpliedByDomCondition(ICmpInst::ICMP_SLT, X, Y, CxtI, DL))
+      return Known;
+  if (match(Op, m_NSWSub(m_Value(X), m_Value(Y))))
+    if (std::optional<bool> Known =
+            isImpliedByDomCondition(ICmpInst::ICMP_SLE, X, Y, CxtI, DL))
+      return Known;
 
   return isImpliedByDomCondition(
       ICmpInst::ICMP_SLT, Op, Constant::getNullValue(Op->getType()), CxtI, DL);


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