[PATCH] eliminate calls to snprintf(0, 0, ...) with known return value (pr78476)

[Jeff Law]

On 11/22/2016 08:02 PM, Martin Sebor wrote:
> Calls to bounded functions like snprintf with a zero-size buffer
> are special requests to compute the size of output without actually
> writing any.  For example:
>
>   int n = snprintf(0, 0, "%08x", rand ());
>
> is a request to compute the number of bytes that the function would
> format if it were passed a buffer of non-zero size.  In the example
> above since the return value is known to be exactly 8, not only can
> the snprintf return value be folded into a constant but the whole
> call to snprintf can be eliminated.
>
> The attached patch enables this optimization under the
> -fprintf-return-value option.  The patch depends on the one for bug
> 78461 (posted earlier today) during the testing of which I noticed
> that this optimization was missing from the gimple-ssa-sprintf pass.
>
> Thanks
> Martin
>
> gcc-78476.diff
>
>
> PR tree-optimization/78476 - snprintf(0, 0, ...) with known arguments not optimized away
>
> gcc/testsuite/ChangeLog:
>
> 	PR tree-optimization/78476
> 	* gcc.dg/tree-ssa/builtin-sprintf-5.c: New test.
>
> gcc/ChangeLog:
>
> 	PR tree-optimization/78476
> 	* gimple-ssa-sprintf.c (struct pass_sprintf_length::call_info):
> 	Add a member.
> 	(handle_gimple_call): Adjust signature.
> 	(try_substitute_return_value): Remove calls to bounded functions
> 	with zero buffer size whose result is known.
> 	(pass_sprintf_length::execute): Adjust call to handle_gimple_call.
OK.

I was initially a little surprised you didn't just delete the call from 
the IL and emit the trivial constant assignment.  Then I realized you 
probably would need to update the vuse/vdefs that were on the original 
call.  Using update_call_from_tree seems to take care of that for you.

I didn't see a negative test -- ie one that you shouldn't transform. 
Could you extract the one from c#0 in the referenced bz and add it as a 
negative test.  You can do that as a separate patch which you should 
consider pre-approved.

Jeff