[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 --- Comment #8 from Andrew Pinski --- (In reply to Eyal Rozenberg from comment #6) > (In reply to Richard Biener from comment #5) > > DSE part ... DCE > > DSE = Dead Statement Elimination? DCE = Dead Code Elimination? I thought Dse was dead store elimination.
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 --- Comment #7 from rguenther at suse dot de --- On Tue, 24 Mar 2020, eyalroz at technion dot ac.il wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 > > --- Comment #6 from Eyal Rozenberg --- > (In reply to Richard Biener from comment #5) > > DSE part ... DCE > > DSE = Dead Statement Elimination? DCE = Dead Code Elimination? Exactly.
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 --- Comment #6 from Eyal Rozenberg --- (In reply to Richard Biener from comment #5) > DSE part ... DCE DSE = Dead Statement Elimination? DCE = Dead Code Elimination?
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 Richard Biener changed: What|Removed |Added CC||rguenth at gcc dot gnu.org Status|UNCONFIRMED |NEW Last reconfirmed||2020-03-24 Ever confirmed|0 |1 Keywords||missed-optimization --- Comment #5 from Richard Biener --- For the DSE part there's no operator delete handling in stmt_kills_ref_p. The clobber would need to be handled by DCE which is the one eliding new/delete pairs. Probably both are not too difficult to handle.
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293
--- Comment #4 from Marc Glisse ---
Or just
void f(){
int*p=new int[1];
*p=42;
delete[] p;
}
while it does optimize for
void f(){
int*p=new int;
*p=42;
delete p;
}
because the front-end gives us a clobber before operator delete.
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293 --- Comment #3 from Eyal Rozenberg --- (In reply to Marc Glisse from comment #1) You should probably post that comment on the second, related, bug 94294 - which is about the fact that GCC keeps the new and delete. This one is strictly about the population of the string, given that new and delete are called.
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293
--- Comment #2 from Eyal Rozenberg ---
Note:
The bugs also manifest with this slightly simpler program:
#include
int bar() {
std::string second { "Hey... no small-string optimization for me please!"
};
return 123;
}
See: https://godbolt.org/z/LjmNYi
[Bug tree-optimization/94293] [missed optimization] Useless statements populating local string not removed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94293
--- Comment #1 from Marc Glisse ---
Adding
inline void* operator new(std::size_t n){return __builtin_malloc(n);}
inline void operator delete(void*p)noexcept{__builtin_free(p);}
inline void operator delete(void*p,std::size_t)noexcept{__builtin_free(p);}
lets gcc optimize. Without it, we end up with
_37 = operator new (51);
__builtin_memcpy (_37, "Hey... no small-string optimization for me please!",
50);
MEM[(char_type &)_37 + 50] = 0;
operator delete (_37, 51);
return 123;
I expect DSE (via tree-ssa-alias.c) doesn't know about delete the way it knows
about free and thus doesn't see the stores as dead.
