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

[Richard Biener]

On June 22, 2021 4:33:09 PM GMT+02:00, Qing Zhao <qing.zhao@oracle.com> wrote:
>
>
>> 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.

No, just bugs that will inevitably show up. 

>> 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?

Do this later, if the need arises. 

Richard. 

>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)