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
