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

[Qing Zhao <qing.zhao@oracle.com>]

> 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.

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)