Why doesn't GCC optimize out deletion of null pointers in C++? -


consider simple program: 

int main() {   int* ptr = nullptr;   delete ptr; }

with gcc (7.2), there call instruction regarding operator delete in resulting program. clang , intel compilers, there no such instructions, null pointer deletion optimized out (-o2 in cases). can test here: https://godbolt.org/g/jmdoji.

i wonder whether such optimization can somehow turned on gcc? (my broader motivation stems problem of custom swap vs std::swap movable types, deletion of null pointers can represent performance penalty in second case; see https://stackoverflow.com/a/45689282/580083 details.)

update

to clarify motivation question: if use delete ptr; without if (ptr) guard in move assignment operator , destructor of class, std::swap objects of class yields 3 call instructions gcc. might considerable performance penalty, e.g., when sorting array of such objects.

moreover, can write if (ptr) delete ptr; everywhere, wonder, whether cannot performance penalty well, since delete expression needs check ptr well. but, here, guess, compilers generate single check only.

also, possibility call delete without guard , surprise me, yield different (performance) outcomes.

update

i did simple benchmark, namely sorting objects, invoke delete in move assignment operator , destructor. source here: https://godbolt.org/g/7zguvo

running times of std::sort measured gcc 7.1 , -o2 flag on xeon e2680v3:

there bug in linked code, compares pointers, not pointed values. corrected results follows:

  1. without if guard: 17.6 [s] 40.8 [s],
  2. with if guard: 10.6 [s] 31.5 [s],
  3. with if guard , custom swap: 10.4 [s] 31.3 [s].

these results absolutely consistent across many runs minimal deviation. performance difference between first 2 cases significant , wouldn't "exceedingly rare corner case" code.

according c++14 [expr.delete]/7:

if value of operand of delete-expression not null pointer value, then:

  • [ ...omitted... ]

otherwise, unspecified whether deallocation function called.

so both compilers comply standard, because it's unspecified whether operator delete called deletion of null pointer.

note godbolt online compiler compiles source file without linking. compiler @ stage must allow possibility operator delete replaced source file.

as speculated in answer -- gcc may angling consistent behaviour in case of replacement operator delete; implementation mean can overload function debug purposes , break on invocations of delete expression, when happened deleting null pointer.

updated: removed speculation might not practical issue, since op provided benchmarks showing in fact is.


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