c - Why did gcc stop warning about strict aliasing violation from version 7.2?

Question

If I compile this code up to gcc 7.1 with -O2 -Wall (or -O3)

#include <stdio.h>

int main (void)
{
  char array [4] = {0x11,0x22,0x33,0x44};
  unsigned int u32 = *(unsigned int*)array; // intentionally invoking strict aliasing UB here
  printf("%x
", u32);
  return 0;
}

Godbolt

Then I get I diagnostic about the undefined behavior:

warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

This only happens when optimizations are enabled.

In 7.2 this warning disappeared and remains disabled to this day. What happened in 7.2 that made the warning go away? As usual with gcc, the change is completely undocumented - https://gcc.gnu.org/gcc-7/changes.html doesn't say a thing about changes to strict aliasing. Compiling with or without -fstrict-aliasing doesn't seem to matter either.

Was the behavior in 7.2 changed on purpose or is this a bug? Are there any known bug reports?

question from:https://stackoverflow.com/questions/65842647/why-did-gcc-stop-warning-about-strict-aliasing-violation-from-version-7-2

Answer 1

I think gcc's warning logic is treating accesses to a top-level character array via pointers of other types as though it were Standard-defined behavior, even though the Standard does not require such treatment, and even though gcc does not treat pointers which are formed by directly indexing such an array likewise. Change the type of the array to something other than a character type and gcc will squawk. Add a value which gcc cannot recognize as a constant (e.g. write a zero to a volatile object and read it back) to the address and gcc won't squawk with any array type, but won't reliably generate meaningful code either.