[LICM] Allow promotion with non-load/store users

[nikic]

Summary

If there are non-load/store users of the promoted pointer, we currently abort promotion. However, having such users isn't really relevant to the transform. We already separately check that a) there are no instructions that modref the promoted pointer and b) that a pointer capture disables store promotion.

In the affected @test_captured_in_loop test case we have a readnone capture of the promoted pointer, which means that load promotion can be performed (while store promotion cannot).

[stefanp]

This is better than what I initially tried to do in D133426.
LGTM

[reames]

LGTM. Nice find!

llvm/lib/Transforms/Scalar/LICM.cpp

2096

If you're interested, we can be smarter here for some uses. e.g. a argument to a call marked experimental gives us deref facts. (Though that requires guaranteed to execute reasoning - which for some reason we don't use for loads?)

llvm/lib/Transforms/Scalar/LICM.cpp

for (Use &U : ASIV->uses()) {
         // If the store is not guaranteed to execute, we may still get
         // deref info through it.
         if (!DereferenceableInPH) {
           DereferenceableInPH = isDereferenceableAndAlignedPointer(
               Store->getPointerOperand(), Store->getValueOperand()->getType(),
               Store->getAlign(), MDL, Preheader->getTerminator(), DT, TLI);
         }
       } else
-        return false; // Not a load or store.
+        continue; // Not a load or store.
 
       if (!AccessTy)
         AccessTy = getLoadStoreType(UI);
       else if (AccessTy != getLoadStoreType(UI))
         return false;
 
       // Merge the AA tags.
       if (LoopUses.empty()) {

llvm/test/Transforms/LICM/promote-capture.ll

exit:
   ret void
 }
 
 define void @test_captured_in_loop(i32 %len) {
 ; CHECK-LABEL: @test_captured_in_loop(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[COUNT:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    store i32 0, i32* [[COUNT]], align 4
+; CHECK-NEXT:    [[COUNT_PROMOTED:%.*]] = load i32, i32* [[COUNT]], align 4
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    [[C_INC2:%.*]] = phi i32 [ [[COUNT_PROMOTED]], [[ENTRY:%.*]] ], [ [[C_INC1:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[I_NEXT:%.*]], [[LATCH]] ]
 ; CHECK-NEXT:    [[COND:%.*]] = call i1 @cond(i32 [[I]])
 ; CHECK-NEXT:    br i1 [[COND]], label [[IF:%.*]], label [[LATCH]]
 ; CHECK:       if:
-; CHECK-NEXT:    [[C:%.*]] = load i32, i32* [[COUNT]], align 4
-; CHECK-NEXT:    [[C_INC:%.*]] = add i32 [[C]], 1
+; CHECK-NEXT:    [[C_INC:%.*]] = add i32 [[C_INC2]], 1
 ; CHECK-NEXT:    store i32 [[C_INC]], i32* [[COUNT]], align 4
 ; CHECK-NEXT:    call void @capture(i32* [[COUNT]])
 ; CHECK-NEXT:    br label [[LATCH]]
 ; CHECK:       latch:
+; CHECK-NEXT:    [[C_INC1]] = phi i32 [ [[C_INC]], [[IF]] ], [ [[C_INC2]], [[LOOP]] ]
 ; CHECK-NEXT:    [[I_NEXT]] = add nuw i32 [[I]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[I_NEXT]], [[LEN:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[LOOP]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
 entry:
   %count = alloca i32