On Tue, Aug 1, 2023, at 19:51, Bart Van Assche wrote:
> On 8/1/23 07:56, Arnd Bergmann wrote:
>> On Tue, Aug 1, 2023, at 16:23, Naresh Kamboju wrote:
>
> If I change the return type of ufshcd_check_header_layout() from void
> into unsigned int and insert the following at the start of that function:
>
> return ((u8 *)&(struct request_desc_header){ .enable_crypto = 1})[2] != 0x80;
>
> then the compiler shows the following in the output window:
>
> xorl %eax, %eax
>
> In other words, the expression next to the return statement evaluates to zero
> but the same expression does not evaluate to zero in the BUILD_BUG_ON()
> statement. Does this perhaps indicate a compiler bug? And if so, what is the
> appropriate way to fix the build error? Insert an #ifdef/#endif pair inside
> ufshcd_check_header_layout() such that the compile-time checks do not happen
> for gcc version 9 or older?
I played around it some more, and this apparently comes
down to constant-folding in sub-byte bitfields, so in the
older compilers neither the ==0x80 nor the !=0x80 case
can be ruled out because of a missing optimization.
Instead the generated code would try to initialize the
variable at runtime and then do a conditional branch to
the assert, but that of course fails the build.
I'd suggest something like
if (defined(GCC_VERSION) && GCC_VERSION < 100000)
return;
before the assertion, in that case it doesn't evaluate it.
Arnd
