[cfe-dev] [RFC] Call ParseLangArgs for all inputs

[Harald van Dijk via cfe-dev]

On 10/09/2020 21:51, Robinson, Paul wrote:
>> -----Original Message-----
>> From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Harald van
>> Dijk via cfe-dev
>> Sent: Thursday, September 10, 2020 2:31 PM
>> To: cfe-dev at lists.llvm.org
>> Subject: Re: [cfe-dev] [RFC] Call ParseLangArgs for all inputs
>>
>> ping 2
>>
>> On 03/09/2020 12:24, Harald van Dijk via cfe-dev wrote:
>>> ping
>>>
>>> On 27/08/2020 12:17, Harald van Dijk via cfe-dev wrote:
>>>> Hi,
>>>>
>>>> clang/lib/CodeGen/BackendUtil.cpp contains multiple checks for
>>>> attributes of LangOpts. The option processing that sets up LangOpts is
>>>> mostly done by ParseLangArgs, which is skipped for LLVM IR files.
>>>> Because of this, there are code generation differences when the
>>>> -save-temps command line option is used: that command line option
>>>> causes LLVM IR to be emitted to file and a new process to be spawned
>>>> to process that file, which does not process options the same way.
> 
> I have to wonder whether some of those options really belong in the CodeGen
> category, rather than the Lang category.  FP handling and sanitizers (the
> two groups that first caught my eye) really don't seem like LangOpts to me.
> 
> Moving them would be more typing, of course, but I suspect the original
> reason ParseLangArgs isn't used on that path is that language-relevant
> actions should already have been taken by the time we're reading IR back in.

This is true. That may be a misunderstanding of the intent of the 
function, as the name can be read two ways: it can be read as processing 
the options that are relevant for the language handling, and it can be 
read as processing the options that are *only* relevant for the language 
handling. The FP options are definitely relevant for the language 
handling and later codegen both. The sanitizers I'm not sure about, they 
are used during language handling, but I do not really know how they are 
used during later codegen. I had thought it would not be worth the 
effort to move the options, but if that would be the preferred route, I 
can submit that as a patch instead. It would have to be limited to the 
options where I can actually verify they affect codegen though, so it 
may be less complete.

> Just a thought.  Offhand I don't know who would be super invested in this.

The reason I was looking at this was to fix an issue reported by a 
customer who was trying to reproduce a bug reported to them. Bug 
handling by them is often done on LLVM IR files, so having different 
behaviour between -save-temps and not -save-temps, and the possibility 
of bugs that only show up when -save-temps is not used, was a problem 
for them. I do not know how common this is in other organisations using 
LLVM, I would think they are not the only one doing this, but it may be 
true that it is only a problem for a minority of users.

Regardless, I'm willing to do the work in getting this fixed, I just 
need some guidance on the preferred way of fixing it.

Cheers,
Harald van Dijk