https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62112
Bug ID: 62112
Summary: Optimize out malloc when block is unused or write-only
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zackw at panix dot com
This program
#include <string.h>
#include <stdlib.h>
int
main(void)
{
size_t n = 1000;
float *x = calloc(n,sizeof(float));
float *y = malloc(n*sizeof(float));
if (x && y)
memcpy(y,x,sizeof(float)*n);
return 0;
}
can be optimized (in the absence of `-fno-builtin-(memcpy|malloc|calloc)`) to
int main(void) { return 0; }
because: the memory block pointed to by `y` is write-only, so the `memcpy` and
`malloc` can be discarded; after that is done, the memory block pointed to by
`x` is unused, so that allocation can be discarded as well.
`calloc` is used here to avoid any question of UB due to reading uninitialized
memory even within `memcpy`. The optimization should apply to all
heap-allocation functions, including especially C++ operator new (as long as
the constructor has no side effects outside the just-allocated object).
Clang 3.5 does perform this optimization.
