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Re: [PATCH] rombios: Work around GCC issue 99578
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 18 Aug 2023 10:44:31 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 18 Aug 2023 09:44:59 +0000
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On 18/08/2023 7:50 am, Jan Beulich wrote:
> On 17.08.2023 22:45, Andrew Cooper wrote:
>> GCC 12 objects to pointers derived from a constant:
>>
>> util.c: In function 'find_rsdp':
>> util.c:429:16: error: array subscript 0 is outside array bounds of
>> 'uint16_t[0]' {aka 'short unsigned int[]'} [-Werror=array-bounds]
>> 429 | ebda_seg = *(uint16_t *)ADDR_FROM_SEG_OFF(0x40, 0xe);
>> cc1: all warnings being treated as errors
>>
> Yet supposedly the bug was fixed in 12.1 (and the fix also backported to
> 11.3). Did you spot anything in ADDR_FROM_SEG_OFF() and this particular
> use of it that is different from the patterns mentioned in that bug?
$ gcc --version
gcc (GCC) 12.2.1 20221121
At a guess, only a partial fix was backported into 12.1. AIUI, it was
an area of logic which had already seen significant development in 13
before the behaviour was reverted.
The only thing interesting about ADDR_FROM_SEG_OFF() is the constant
folding required for the expression to become *(uint16_t *)0x40e, which
(I suspect) is why it compiles fine at -Og but fails at -O2.
>> This is a GCC bug, but work around it rather than turning array-bounds
>> checking off generally.
> I certainly agree here. I guess it's not worth trying to restrict the
> workaround for rombios (I will want to try doing so in the hypervisor).
Can I translate this to an ack?
~Andrew
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