[PATCH][version 3]add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

Qing Zhao qing.zhao@oracle.com
Tue Jun 22 14:33:09 GMT 2021



> On Jun 22, 2021, at 9:15 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Tue, 22 Jun 2021, Qing Zhao wrote:
> 
>> 
>> 
>>> On Jun 22, 2021, at 9:00 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Tue, 22 Jun 2021, Qing Zhao wrote:
>>> 
>>>> So, I am wondering why not still keep my current implementation on 
>>>> assign different patterns for different types?
>>>> 
>>>> This major issue with this design is the code size and runtime overhead, 
>>>> but for debugging purpose, those are not that important, right? And we 
>>>> can add some optimization later to improve the code size and runtime 
>>>> overhead.
>>>> 
>>>> Otherwise, if we only use one pattern for all the types in this initial 
>>>> version, later we still might need change it.
>>>> 
>>>> How do you think?
>>> 
>>> No, let's not re-open that discussion.  As said we can look to support
>>> multi-byte pattern if that has a chance to improve things but only
>>> as followup.
>> 
>> I am fine with this.
>> 
>> However, we need to decide whether we will use one-byte repeatable pattern, or multiple-byte repeatable pattern now,
>> Since the implementation will be different. If using one-byte, the implementation will be the simplest, we can use memset for all
>> VLA, non-vla, zero-init, or pattern-init consistently.
>> 
>> However, if we choose multiple-byte pattern, then the implementation will be different, we cannot use memset for pattern-init, and 
>> The implemenation for VLA pattern-init also is different.
> 
> As said, we can do this as followup.  For now get the easiest thing
> working - one-byte patterns via memset.  

Okay. I will work on this.

> There's enough bits in the
> patch that will likely need followup fixes (the .DEFERED_INIT stuff),

Do you mean your previous suggestion to merge the handling of VLA to non-VLA during gimplification phase?
I have done with this change locally.

> actual code gneration of the init is separate enough we can deal with
> it later.  Also IMHO not all targets necessarily need to behave the
> same there.

Then, shall we make the code generation part a target hook now? Or do this later?

Qing
> 
> Richard.
> 
>> Qing
>>> 
>>> Thanks,
>>> Richard.
>>> 
>>>> Qing
>>>> 
>>>> On Jun 22, 2021, at 3:59 AM, Richard Biener <rguenther@suse.de<mailto:rguenther@suse.de>> wrote:
>>>> 
>>>> On Tue, 22 Jun 2021, Richard Sandiford wrote:
>>>> 
>>>> Kees Cook <keescook@chromium.org<mailto:keescook@chromium.org>> writes:
>>>> On Mon, Jun 21, 2021 at 03:39:45PM +0000, Qing Zhao wrote:
>>>> So, if “pattern value” is “0xFFFFFFFFFFFFFFFF”, then it’s a valid canonical virtual memory address.  However, for most OS, “0xFFFFFFFFFFFFFFFF” should be not in user space.
>>>> 
>>>> My question is, is “0xFFFFFFFFFFFFFFFFF” good for pointer? Or “0xAAAAAAAAAAAAAAAA” better?
>>>> 
>>>> I think 0xFF repeating is fine for this version. Everything else is a
>>>> "nice to have" for the pattern-init, IMO. :)
>>>> 
>>>> Sorry to be awkward, but 0xFF seems worse than 0xAA to me.
>>>> 
>>>> For integer types, all values are valid representations, and we're
>>>> relying on the pattern being “obviously” wrong in context.  0xAAAA…
>>>> is unlikely to be a correct integer but 0xFFFF… would instead be a
>>>> “nice” -1.  It would be difficult to tell in a debugger that a -1
>>>> came from pattern init rather than a deliberate choice.
>>>> 
>>>> I agree that, all other things being equal, it would be nice to use NaNs
>>>> for floats.  But relying on wrong numerical values for floats doesn't
>>>> seem worse than doing that for integers.
>>>> 
>>>> 0xAA… for float is (if I've got this right) -3.0316488252093987e-13,
>>>> which admittedly doesn't stand out as wrong.  But I'm not sure we
>>>> should sacrifice integer debugging for float debugging here.
>>>> 
>>>> We can always expose the actual value as --param.  Now, I think
>>>> we'd need a two-byte pattern to reliably produce NaNs anyway,
>>>> so with floats taken out of the picture the focus should be on
>>>> pointers where IMHO val & 1 and val & 15 would be nice to have.
>>>> So sth like 0xf7 would work for those.  With a two-byte pattern
>>>> we could use 0xffef or 0x7fef.
>>>> 
>>>> Anyway, it's probably down to priorities of the project involved
>>>> (debugging FP stuff or integer stuff).
>>>> 
>>>> Richard.
>>>> 
>>>> 
>>> 
>>> -- 
>>> Richard Biener <rguenther@suse.de>
>>> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
>>> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)



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