bin/59245: GCC silent data corruption due to "#pragma GCC diagnostic push/pop"

[cryintothebluesky%gmail.com@localhost]

>Number:         59245
>Category:       bin
>Synopsis:       GCC silent data corruption due to "#pragma GCC diagnostic push/pop"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Apr 01 15:55:00 +0000 2025
>Originator:     Sad Clouds
>Release:        
>Organization:
>Environment:
>Description:
I'm observing some weird corruption with GCC on sparc64 running
NetBSD-9.4 and evbarmv7hf-el running NetBSD-9.2.

The corruption seems to occur with the following pragmas:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wcast-align"
...

#pragma GCC diagnostic pop

This results in u64_ptr pointer corruption, which points to the wrong
address and prints the wrong values in both cases:

$ gcc test.c && ./a.out
data=0xffffffffffffd388, u64_ptr=0xffffffffffffd390, *u64_ptr=1078020256
data=0xffffffffffffd380, u64_ptr=0xffffffffffffd388, *u64_ptr=1122334455

If I comment/remove GCC pragmas, the code executes correctly. Pointers:
data and u64_ptr are the same and the values printed are also correct:

$ gcc test.c && ./a.out
data=0xffffffffffffd388, u64_ptr=0xffffffffffffd388, *u64_ptr=1122334455
data=0xffffffffffffd380, u64_ptr=0xffffffffffffd380, *u64_ptr=6677889900

The following test program can be used to reproduce the issue


#include <assert.h>
#include <stdio.h>
#include <stdint.h>

static void fn(const void *data, size_t data_size)
{
	const uint64_t *u64_ptr;

	/* data must be aligned on 8 byte boundary */
	assert((uintptr_t)data % sizeof(uint64_t) == 0);
	assert(data_size >= sizeof(uint64_t));
	assert(data_size % sizeof(uint64_t) == 0);

	#pragma GCC diagnostic push
	#pragma GCC diagnostic ignored "-Wcast-align"
	for (
		u64_ptr = data;
		u64_ptr < (const uint64_t *)((const uint8_t *)data + data_size);
		u64_ptr++)
	#pragma GCC diagnostic pop
	{
		printf("data=%p, u64_ptr=%p, *u64_ptr=%ju\n", data, u64_ptr, *u64_ptr);
	}
}

int main(void)
{
	uint64_t n1 = 1122334455;
	uint64_t n2 = 6677889900;

	fn(&n1, sizeof(n1));
	fn(&n2, sizeof(n2));
}
>How-To-Repeat:

>Fix: