linux-kernel - Re: next: arm64: gcc-8-defconfig: ufshcd.c:10629:2: /builds/linux/include/linux/compiler_types.h:397:38: error: call to '__compiletime_assert_553' declared with attribute error: BUILD_BUG

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Date:   Tue, 01 Aug 2023 21:54:06 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Bart Van Assche" <bvanassche@....org>,
        "Naresh Kamboju" <naresh.kamboju@...aro.org>
Cc:     "open list" <linux-kernel@...r.kernel.org>,
        linux-scsi@...r.kernel.org,
        linux-next <linux-next@...r.kernel.org>,
        "Avri Altman" <avri.altman@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Anders Roxell" <anders.roxell@...aro.org>
Subject: Re: next: arm64: gcc-8-defconfig: ufshcd.c:10629:2:
 /builds/linux/include/linux/compiler_types.h:397:38: error: call to
 '__compiletime_assert_553' declared with attribute error: BUILD_BUG_ON
 failed:

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