llvm dev - Jan 2013 - [LLVMdev] Having trouble with GEP

On Wed, Jan 16, 2013 at 8:22 AM, Tim Northover <t.p.northover at
gmail.com> wrote:
> I get that error when the %str type is undefined (presumably because
> LLVM doesn't know its alignment requirements so can't lay out the
> array$str type properly). For example, the following complete program
> works correctly for me:
>
> %str = type {i8*, i64}
> %array$str = type { i64, [0 x %str] }
>
> declare i8* @malloc(i64)
>
> define i64 @foo(i64 %objsz) {
>   %array.raw = call i8* @malloc(i64 %objsz)
>   %array = bitcast i8* %array.raw to %array$str*
>   %array.data = getelementptr %array$str* %array, i32 0, i32 1
>   %ret = ptrtoint [0 x %str]* %array.data to i64
>   ret i64 %ret
> }
>
> But it fails if the first line is commented out. Could you give a
> similar complete example that has the error you're seeing?
Yes:

%array$str = type { i64, [0 x %str] }

declare i8* @malloc(i64)

define i64 @foo(i64 %objsz) {
    %array.raw = call i8* @malloc(i64 %objsz);
    %array = bitcast i8* %array.raw to %array$str*
    %array.data = getelementptr %array$str* %array, i32 0, i32 1
    %ret = ptrtoint [0 x %str]* %array.data to i64
    ret i64 %ret
}

%str = type {i8*, i64}

So you're right that it does have to do with the %str type not being
defined.

What I'm confused about is why LLVM isn't able to generate a better
error message than this. Perhaps this is one of those cases that
hasn't been tested much because not many people write their IR without
making use of LLVM's API, which would error out sooner if I tried
this? Or is there a more fundamental reason why there's no better
error message? Is it useful to create a bug about this?

Cheers,

Dirkjan

> Yes:
>
> %array$str = type { i64, [0 x %str] }
>
> declare i8* @malloc(i64)
>
> define i64 @foo(i64 %objsz) {
>     %array.raw = call i8* @malloc(i64 %objsz);
>     %array = bitcast i8* %array.raw to %array$str*
>     %array.data = getelementptr %array$str* %array, i32 0, i32 1
>     %ret = ptrtoint [0 x %str]* %array.data to i64
>     ret i64 %ret
> }
>
> %str = type {i8*, i64}
>
> So you're right that it does have to do with the %str type not being
defined.
>
> What I'm confused about is why LLVM isn't able to generate a better
> error message than this. Perhaps this is one of those cases that
> hasn't been tested much because not many people write their IR without
> making use of LLVM's API, which would error out sooner if I tried
> this? Or is there a more fundamental reason why there's no better
> error message? Is it useful to create a bug about this?
I think it makes sense to create a bug report. Looking at the code of
LLParser::ParseGetElementPtr, the error handling appears to be very
simplistic, so I don't think there's any fundamental reason for not
doing a better job there.

Eli

On Wed, Jan 16, 2013 at 5:18 PM, Eli Bendersky <eliben at google.com>
wrote:
> I think it makes sense to create a bug report. Looking at the code of
> LLParser::ParseGetElementPtr, the error handling appears to be very
> simplistic, so I don't think there's any fundamental reason for not
> doing a better job there.
Filed http://llvm.org/bugs/show_bug.cgi?id=14965, thanks!

Cheers,

Dirkjan