[PATCH] run early sprintf warning after SSA (PR 100325)

[Aldy Hernandez]

On 5/5/21 9:26 AM, Richard Biener wrote:
> On Wed, May 5, 2021 at 1:32 AM Martin Sebor via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> With no optimization, -Wformat-overflow and -Wformat-truncation
>> runs early to detect a subset of simple bugs.  But as it turns out,
>> the pass runs just a tad too early, before SSA.  That causes it to
>> miss a class of problems that can easily be detected once code is
>> in SSA form, and I would expect might also cause false positives.
>>
>> The attached change moves the sprintf pass just after pass_build_ssa,
>> similar to other early flow-sensitive warnings (-Wnonnull-compare and
>> -Wuninitialized).
> 
> Makes sense.  I suppose walloca might also benefit from SSA - it seems
> to do range queries which won't work quite well w/o SSA?

The early Walloca pass that runs without optimization doesn't do much, 
as we've never had ranges so early.  All it does is diagnose _every_ 
call to alloca(), if -Walloca is passed:

   // The first time this pass is called, it is called before
   // optimizations have been run and range information is unavailable,
   // so we can only perform strict alloca checking.
   if (first_time_p)
     return warn_alloca != 0;

Though, I suppose we could move the first alloca pass after SSA is 
available and make it the one and only pass, since ranger only needs 
SSA.  However, I don't know how well this would work without value 
numbering or CSE.  For example, for gcc.dg/Walloca-4.c the gimple is:

   <bb 2> :
   _1 = rear_ptr_9(D) - w_10(D);
   _2 = (long unsigned int) _1;
   if (_2 <= 4095)
     goto <bb 3>; [INV]
   else
     goto <bb 4>; [INV]

   <bb 3> :
   _3 = rear_ptr_9(D) - w_10(D);
   _4 = (long unsigned int) _3;
   src_16 = __builtin_alloca (_4);
   goto <bb 5>; [INV]

No ranges can be determined for _4.  However, if either FRE or DOM run, 
as they do value numbering and CSE respectively, we could easily 
determine a range as the above would become:

  <bb 2> :
   _1 = rear_ptr_9(D) - w_10(D);
   _2 = (long unsigned int) _1;
   if (_2 <= 4095)
     goto <bb 3>; [INV]
   else
     goto <bb 4>; [INV]

   <bb 3> :
   src_16 = __builtin_alloca (_2);
   goto <bb 5>; [INV]

I'm inclined to leave the first alloca pass before SSA runs, as it 
doesn't do anything with ranges.  If anyone's open to a simple -O0 CSE 
type pass, it would be a different story.  Thoughts?

Aldy