(original) (raw)

Hi all,

Let's see an example (from Alexandre Isoard) first:

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; RUN: opt -ipsccp -deadargelim -licm -O2 -S < %s

@g = internal global i32 0

; Function Attrs: argmemonly

define internal void @foo(i32\* nonnull dereferenceable(4) %arg, i32 %val) #0 {

entry:

store i32 %val, i32\* %arg

ret void

}

define i32 @bar(i32 %n) {

entry:

store i32 1, i32\* @g

br label %loop

loop: ; preds = %bb1, %bb

%i = phi i32 \[ %i.inc, %loop \], \[ 0, %entry \]

%g.val = load i32, i32\* @g

%g.inc = add nuw i32 %g.val, 1

tail call void @foo(i32\* @g, i32 %g.inc)

%i.inc = add nuw i32 %i, 1

%cond = icmp eq i32 %i.inc, %n

br i1 %cond, label %exit, label %loop

exit: ; preds = %bb1

ret i32 %g.val

}

declare void @llvm.assume(i1)

attributes #0 = { argmemonly }

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With opt cmd '-ipsccp -deadargelim -licm -O2', function @bar will return constant value 1, instead of correct value %n. This is crazy, right?

Let's see what happens here.

Due to pass 'ipsccp' replaced the argument of function '@foo' with global variable '@g', but keeps attr 'argmemonly' there, so pass 'licm' will hoist the load of '@g' before the loop (as the value of @g isn't changed, but it is changed), and will cause function return wrong constant value '1', instead of '%n' (the correct value) , like following:

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; Function Attrs: nofree norecurse nounwind writeonly

define i32 @bar(i32 %n) local\_unnamed\_addr #0 {

entry:

ret i32 1

}

attributes #0 = { nofree norecurse nounwind writeonly }

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And if remove the 'argmemonly' attribute, function @bar will return the correct value:

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; Function Attrs: nofree norecurse nounwind

define i32 @bar(i32 %n) local\_unnamed\_addr #0 {

entry:

ret i32 %n

}

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So if function attribute 'argmemonly' on function @foo is correct, then there's a bug in pass 'ipsccp'. When it replaces the function local value with global variable, then it shoud remember to remove this function attribute.

Thanks,

Fangqing