Re: [PATCH 4/5] sim/erc32: avoid dereferencing type-punned pointer warnings

Pedro Alves
2022-10-12T14:11:47Z

On 2022-10-12 1:38 p.m., Andrew Burgess via Gdb-patches wrote:
> When building the erc32 simulator I get a few warnings like this:
>
> /tmp/build/sim/../../src/sim/erc32/exec.c:1377:21: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> 1377 | sregs->fs[rd] = *((float32 *) & ddata[0]);
> | ~^~~~~~~~~~~~~~~~~~~~~~~
>
> The type of '& ddata[0]' will be 'uint32_t *', which is what triggers
> the warning.
>
> This commit uses an intermediate pointer of type 'char *' when
> performing the type-punning, which is well-defined behaviour, and will
> silence the above warning.

I'm afraid that's not correct. That's still undefined behavior, it's just silencing
the warning. The end result is still aliasing float32 and uint32_t objects, and risks
generating bogus code. The char escape hatch only works if you access the object
representation via a character type. Here, the patch is still accessing the object
representation of a uint32_t object via a floa32 type.

Here's an old article explaining strict aliasing (just one that I found via a quick google):

https://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html

This scenario is the "CASTING TO CHAR*" one in that article.

> @@ -1345,7 +1345,8 @@ dispatch_instruction(struct pstate *sregs)
> if (mexc) {
> sregs->trap = TRAP_DEXC;
> } else {
> - sregs->fs[rd] = *((float32 *) & data);
> + char *ptr = (char *) &data;
> + sregs->fs[rd] = *((float32 *) ptr);

The best IMHO is to do the type punning via a union, like e.g.:

union { float32 f; uint32_t i; } u;
u.i = data;
sregs->fs[rd] = u.f;

See here in the C11 standard:

https://port70.net/~nsz/c/c11/n1570.html#note95

and also the documentation of -fstrict-aliasing at:

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Pedro Alves