[PATCH] D150378: [Instsimplfy] X == Y ? 0 : X - Y --> X - Y

[Jun Zhang via Phabricator via llvm-commits]

junaire updated this revision to Diff 522533.
junaire added a comment.

Address comments, thanks!


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D150378/new/

https://reviews.llvm.org/D150378

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/test/Transforms/InstSimplify/select.ll


Index: llvm/test/Transforms/InstSimplify/select.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/select.ll
+++ llvm/test/Transforms/InstSimplify/select.ll
@@ -1441,10 +1441,8 @@
 
 define i8 @select_sub_cmp(i8 %0, i8 %1) {
 ; CHECK-LABEL: @select_sub_cmp(
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i8 [[TMP1:%.*]], [[TMP0:%.*]]
-; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw i8 [[TMP1]], [[TMP0]]
-; CHECK-NEXT:    [[TMP5:%.*]] = select i1 [[TMP3]], i8 0, i8 [[TMP4]]
-; CHECK-NEXT:    ret i8 [[TMP5]]
+; CHECK-NEXT:    [[TMP3:%.*]] = sub nsw i8 [[TMP1:%.*]], [[TMP0:%.*]]
+; CHECK-NEXT:    ret i8 [[TMP3]]
 ;
   %3 = icmp eq i8 %1, %0
   %4 = sub nsw i8 %1, %0
@@ -1454,10 +1452,8 @@
 
 define <2 x i8> @select_sub_cmp_vec(<2 x i8> %0, <2 x i8> %1) {
 ; CHECK-LABEL: @select_sub_cmp_vec(
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq <2 x i8> [[TMP1:%.*]], [[TMP0:%.*]]
-; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw <2 x i8> [[TMP1]], [[TMP0]]
-; CHECK-NEXT:    [[TMP5:%.*]] = select <2 x i1> [[TMP3]], <2 x i8> zeroinitializer, <2 x i8> [[TMP4]]
-; CHECK-NEXT:    ret <2 x i8> [[TMP5]]
+; CHECK-NEXT:    [[TMP3:%.*]] = sub nsw <2 x i8> [[TMP1:%.*]], [[TMP0:%.*]]
+; CHECK-NEXT:    ret <2 x i8> [[TMP3]]
 ;
   %3 = icmp eq <2 x i8> %1, %0
   %4 = sub nsw <2 x i8> %1, %0
@@ -1467,10 +1463,8 @@
 
 define i8 @select_sub_cmp_swap(i8 %0, i8 %1) {
 ; CHECK-LABEL: @select_sub_cmp_swap(
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i8 [[TMP1:%.*]], [[TMP0:%.*]]
-; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw i8 [[TMP0]], [[TMP1]]
-; CHECK-NEXT:    [[TMP5:%.*]] = select i1 [[TMP3]], i8 0, i8 [[TMP4]]
-; CHECK-NEXT:    ret i8 [[TMP5]]
+; CHECK-NEXT:    [[TMP3:%.*]] = sub nsw i8 [[TMP0:%.*]], [[TMP1:%.*]]
+; CHECK-NEXT:    ret i8 [[TMP3]]
 ;
   %3 = icmp eq i8 %1, %0
   %4 = sub nsw i8 %0, %1
@@ -1480,10 +1474,8 @@
 
 define <2 x i8> @select_sub_cmp_vec_swap(<2 x i8> %0, <2 x i8> %1) {
 ; CHECK-LABEL: @select_sub_cmp_vec_swap(
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq <2 x i8> [[TMP1:%.*]], [[TMP0:%.*]]
-; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw <2 x i8> [[TMP0]], [[TMP1]]
-; CHECK-NEXT:    [[TMP5:%.*]] = select <2 x i1> [[TMP3]], <2 x i8> zeroinitializer, <2 x i8> [[TMP4]]
-; CHECK-NEXT:    ret <2 x i8> [[TMP5]]
+; CHECK-NEXT:    [[TMP3:%.*]] = sub nsw <2 x i8> [[TMP0:%.*]], [[TMP1:%.*]]
+; CHECK-NEXT:    ret <2 x i8> [[TMP3]]
 ;
   %3 = icmp eq <2 x i8> %1, %0
   %4 = sub nsw <2 x i8> %0, %1
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4247,6 +4247,13 @@
           NewOps[0] == NewOps[1])
         return NewOps[0];
 
+      // x - x -> 0. This is non-refining, because x is non-poison by assumption
+      // and this case never wraps, so nowrap flags can be ignored.
+      if (Opcode == Instruction::Sub && NewOps[0] == NewOps[1]) {
+        assert(NewOps[0] == RepOp && "Precondition for non-poison assumption");
+        return Constant::getNullValue(I->getType());
+      }
+
       // If we are substituting an absorber constant into a binop and extra
       // poison can't leak if we remove the select -- because both operands of
       // the binop are based on the same value -- then it may be safe to replace


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