On Sat, 28 Aug 2021, Jeremy Sawicki wrote:
> I am attempting to access the x86_64 DIV instruction from inline
> assembly as follows:
>
> inline void div128(uint64_t* q, uint64_t *r,
> uint64_t n1, uint64_t n0, uint64_t d)
> {
> __asm__("divq %4"
> : "=a" (*q), "=d" (*r)
> : "1" (n1), "0" (n0), "rm" (d)
> : "cc");
> }
>
> I declare input and output operands, but is that sufficient? The DIV
> instruction can produce a divide error for certain inputs. The GCC
> documentation says that inline assembly may be discarded or moved out
> of loops. Can it also be speculatively executed? I have a case in
> GCC 11.2 where a single DIV is duplicated and executed on two different
> inputs coming from two sides of a conditional. One of the inputs (the
> one not chosen by the conditional) produces a divide error.
>
> Is my code at fault? Do I need the volatile qualifier? Or is GCC not
> supposed to speculatively execute inline assembly?
Since your code takes care to invoke 'div128' only with safe inputs, it's
fine. GCC has no way to distinguish potentially-trapping asm statements,
so to be conservatively correct it may not move them speculatively.
Can you file your testcase in the Bugzilla? It might be a good idea to
reference these existing bug reports:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82677
(general discussion of the issue you're facing)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93491
(not your issue, but related in the sense that pure/const functions
need to be non-trapping only for the inputs they receive in the abstract
machine -- same applies to inline asms)
Alexander
