Bug#1063882: gcc: Internal error from ternary cond as inline asm parameter

[some body]

On Wed, 14 Feb 2024 08:26:41 +0100 Matthias Klose  wrote:
> Control: tags -1 + moreinfo
>
> On 14.02.24 02:10, VictorBW wrote:
> > Package: gcc
> > Version: 4:12.2.0-3
> > Severity: normal
> > X-Debbugs-Cc: knodewaeee+debb...@gmail.com
> >
> > Dear Maintainer,
> >
> > I wanted to dynamically select registers for use in an inline assembly
statement, so I tried the questionmark conditional operator, as in this
minimal example:
> >
> > namespace gpr {
> >volatile register int64_t r12 asm("r12");
> >volatile register int64_t r13 asm("r13");
> > ...
> > asm volatile ("mov %0, blah" : "+r"((reg) ? gpr::r13 : gpr::r12));
> >
>
> please post the complete code example.
>
> please also recheck with newer GCC versions (GCC 13, GCC 14) in newer
> Debian development versions.
>
I do not have GCC 13/14 installed, but godbolt.org's copy of GCC 13 and
trunk will not compile the following complete example

int main(int argc, char** argv) {
register long reg_1 asm("r12");
register long reg_2 asm("r13");
asm volatile ("mov %0, %%r11" : "+r"((argc) ? reg_2 : reg_1));
}

I note that in C mode (gcc minbug.c) a different error is produced (error:
lvalue required in ‘asm’ statement), but in C++ mode (g++ minbug.c) we get
the internal compiler error, and that selecting an input parameter the same
way (rather than an output parameter) seems to work correctly.

Bug#1063882: gcc: Internal error from ternary cond as inline asm parameter

[Matthias Klose]

Control: tags -1 + moreinfo

On 14.02.24 02:10, VictorBW wrote:

Package: gcc
Version: 4:12.2.0-3
Severity: normal
X-Debbugs-Cc: knodewaeee+debb...@gmail.com

Dear Maintainer,

I wanted to dynamically select registers for use in an inline assembly 
statement, so I tried the questionmark conditional operator, as in this minimal 
example:

namespace gpr {
volatile register int64_t r12 asm("r12");
volatile register int64_t r13 asm("r13");
...
asm volatile ("mov %0, blah" : "+r"((reg) ? gpr::r13 : gpr::r12));



please post the complete code example.

please also recheck with newer GCC versions (GCC 13, GCC 14) in newer 
Debian development versions.



This generates an internal error:
$ g++ bug.cpp
during RTL pass: expand
bug.cpp: In function ‘void move(uint8_t, int64_t)’:
bug.cpp:11:47: internal compiler error: in expand_expr_addr_expr_1, at 
expr.cc:8435
   11 | asm volatile ("mov %0, blah" : "+r"((reg) ? gpr::r13 : 
gpr::r12));
  | 
~~^

g++ -freport-bug did not deem the bug reproducible, but godbolt.org produces 
the following backtrace
0x264bdbc internal_error(char const*, ...)
???:0
0xa523e3 fancy_abort(char const*, int, char const*)
???:0
0xf62e6e expand_expr_real_1(tree_node*, rtx_def*, machine_mode, 
expand_modifier, rtx_def**, bool)
???:0
0xf703ae store_expr(tree_node*, rtx_def*, int, bool, bool)
???:0

I expected:
Realistically, a proper compiler error
Optimistically, generation of appropriate branches

I will apply an alternative solution in the meantime, but the latter behaviour 
would be very nice to have.

Perhaps other builtins also generate improper errors when used with a 
conditional operator? (I have not tried)

First time Debian/GCC bugreport, please forgive any relevant blunders :)

-- System Information:
Debian Release: 12.5
   APT prefers stable-updates
   APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 
'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386, arm64