Hello,
On Fri, 13 Dec 2019, Giovanni Mascellani wrote:
> TinyCC fails to test on i386 on the CI that I just set up. Apparently,
> though, the problem is not in tcc, but in gcc, which is used to compile
> the test program too and compare the results.
>
> I managed to trim down the offending example to this:
>
> int main() {
> asm volatile ("mov %%eax, %%eax\n" : : "i" ("A string"));
> return 0;
> }
>
> If I compile this with gcc (9.2) on Debian this happens:
>
> $ gcc -g -m32 test.c
> test.c: In function ‘main’:
> test.c:3:5: warning: asm operand 0 probably doesn’t match constraints
> 3 | asm volatile ("mov %%eax, %%eax\n" : : "i" ("A string"));
> | ^~~
> test.c:3:5: error: impossible constraint in ‘asm’
>
> I believe this is wrong, "i" should accept a literal string as value.
> Also, the same program works on Compiler Explorer[1].
>
> [1] https://godbolt.org/z/xAP-dy
>
> I suspect this might be a bug in Debian's gcc. Anybody not using Debian
> could please try to compile this program and see what happens?
I had now time to look into this. The immediate cause is that new Debian
enables PIC by default also with -m32, which triggers the error message.
It was always there, i.e. the construct used in the kernel is only usable
with either x86-64 or without -fPIC/-fPIE on i386. There are ways to
write this construct without triggering the error message, but I want to
test the thing as written in the kernel. So the only option besides
disabling the test is to force non-PIC code for it on i386, which
means for the whole file. I've done that now on mob with a lengthy
comment in tcctest.c.
Oh, and the testing pipeline is useful, thanks for setting it up :-)
(Though it would be nice if at least the native machines, i.e.
i386 and x86-64, wouldn't use a full system emulation seemingly without
paravirt; 8 minutes for a testrun of tcc seems a bit excessive; but as you
said, definitely better than nothing :) )
Ciao,
Michael._______________________________________________
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