[PATCH] D127499: [InstCombine] Don't push operation across loop phi

[Nikita Popov via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG92a9b1c9184c: [InstCombine] Don't push operation across loop phi (authored by nikic).

Changed prior to commit:
  https://reviews.llvm.org/D127499?vs=435907&id=436296#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127499/new/

https://reviews.llvm.org/D127499

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/cast_phi.ll


Index: llvm/test/Transforms/InstCombine/cast_phi.ll
===================================================================
--- llvm/test/Transforms/InstCombine/cast_phi.ll
+++ llvm/test/Transforms/InstCombine/cast_phi.ll
@@ -320,15 +320,15 @@
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
-; CHECK-NEXT:    [[PHI:%.*]] = phi i8 [ 1, [[ENTRY]] ], [ [[PHI_CAST:%.*]], [[LOOP_LATCH]] ]
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[IV_NEXT]], [[LOOP_LATCH]] ]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[IV]], 100
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT:%.*]]
 ; CHECK:       loop.latch:
 ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
-; CHECK-NEXT:    [[PHI_CAST]] = trunc i32 [[IV_NEXT]] to i8
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
-; CHECK-NEXT:    ret i8 [[PHI]]
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[PHI]] to i8
+; CHECK-NEXT:    ret i8 [[TRUNC]]
 ;
 entry:
   br label %loop
@@ -353,16 +353,16 @@
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[PHI_CAST:%.*]], [[LOOP_LATCH:%.*]] ]
-; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i32 [[IV]], [[END:%.*]]
+; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[IV_EXT:%.*]] = zext i8 [[IV]] to i32
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i32 [[IV_EXT]], [[END:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[IV_TR:%.*]] = trunc i32 [[IV]] to i8
-; CHECK-NEXT:    [[IV_NEXT_NARROW:%.*]] = add i8 [[IV_TR]], [[STEP:%.*]]
-; CHECK-NEXT:    [[PHI_CAST]] = zext i8 [[IV_NEXT_NARROW]] to i32
+; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], [[STEP:%.*]]
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
-; CHECK-NEXT:    ret i32 [[IV]]
+; CHECK-NEXT:    [[EXT:%.*]] = zext i8 [[IV]] to i32
+; CHECK-NEXT:    ret i32 [[EXT]]
 ;
 entry:
   br label %loop
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1177,10 +1177,11 @@
       if (cast<Instruction>(InVal)->getParent() == NonConstBB)
         return nullptr;
 
-    // If the incoming non-constant value is in I's block, we will remove one
-    // instruction, but insert another equivalent one, leading to infinite
-    // instcombine.
-    if (isPotentiallyReachable(I.getParent(), NonConstBB, nullptr, &DT, LI))
+    // If the incoming non-constant value is reachable from the phis block,
+    // we'll push the operation across a loop backedge. This could result in
+    // an infinite combine loop, and is generally non-profitable (especially
+    // if the operation was originally outside the loop).
+    if (isPotentiallyReachable(PN->getParent(), NonConstBB, nullptr, &DT, LI))
       return nullptr;
   }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D127499.436296.patch
Type: text/x-patch
Size: 3173 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20220613/d26064e2/attachment.bin>