[Bug inline-asm/65640] multiple alternative constraints and earlyclobbers
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65640
Segher Boessenkool changed:
What|Removed |Added
CC||segher at gcc dot gnu.org
--- Comment #3 from Segher Boessenkool ---
(Please don't quote from previous messages when not replying to those lines).
This only fails with -O0.
IRA assigns the following:
int f(int out, int in) {
asm("foo %1,%0;" : "=" (out) : "b" (in));// a a
asm("foo %1,%0;" : "=" (out) : "b" (in));// a b
asm("foo %1,%0;" : "=" (out) : "a" (in));// a b
asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // a b
asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // a b
asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // a b
asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // a b
return out;
}
The LRA is left to clean up after it, and it comes up with
int f(int out, int in) {
asm("foo %1,%0;" : "=" (out) : "b" (in));// a b
asm("foo %1,%0;" : "=" (out) : "b" (in));// - -
asm("foo %1,%0;" : "=" (out) : "a" (in));// - -
asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // a b
asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // - -
asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // a b
asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // b a
return out;
}
(The "- -" are when LRA couldn't come up with something that works).
[Bug inline-asm/65640] multiple alternative constraints and earlyclobbers
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65640
--- Comment #2 from Eric Gallager ---
(In reply to Eric Gallager from comment #1)
> (In reply to Mike Herrick from comment #0)
> > There seems to be an issue with earlyclobbers and multiple alternative
> > constraints. Take this example:
> >
> > int f(int out, int in) {
> > asm("foo %1,%0;" : "=" (out) : "b" (in));// Okay
> > asm("foo %1,%0;" : "=" (out) : "b" (in));// Expected error
> > asm("foo %1,%0;" : "=" (out) : "a" (in));// Expected error
> > asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // Okay
> > asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Unexpected error
> > asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // Okay
> > asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Okay
> > return out;
> > }
> >
> > With 4.9.2, I see three errors:
> >
> > $ g++492 -S ex.c
> > ex.c: In function 'int f(int, int)':
> > ex.c:3:47: error: 'asm' operand has impossible constraints
> > asm("foo %1,%0;" : "=" (out) : "b" (in));// Expected error
> >^
> > ex.c:4:47: error: 'asm' operand has impossible constraints
> > asm("foo %1,%0;" : "=" (out) : "a" (in));// Expected error
> >^
> > ex.c:6:52: error: 'asm' operand has impossible constraints
> > asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Unexpected error
> > ^
> >
> > The first two are expected, but I'm at a loss to explain the third error.
> > In all four of the multi alternative constrain cases, there is exactly one
> > "good" constraint and one "bad" constraint in the pair, yet only one of the
> > four cases is diagnosed.
> >
> > What is the expected behavior for this case?
>
> Confirmed that I get errors in the same places as you; I'm not sure exactly
> what the expected behavior is though. Maybe someone else can answer.
Any ideas on who else could answer? There's no "inline-asm" maintainer listed
in the MAINTAINERS file...
[Bug inline-asm/65640] multiple alternative constraints and earlyclobbers
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65640
Eric Gallager changed:
What|Removed |Added
Status|UNCONFIRMED |NEW
Last reconfirmed||2017-07-28
CC||egallager at gcc dot gnu.org
Ever confirmed|0 |1
--- Comment #1 from Eric Gallager ---
(In reply to Mike Herrick from comment #0)
> There seems to be an issue with earlyclobbers and multiple alternative
> constraints. Take this example:
>
> int f(int out, int in) {
> asm("foo %1,%0;" : "=" (out) : "b" (in));// Okay
> asm("foo %1,%0;" : "=" (out) : "b" (in));// Expected error
> asm("foo %1,%0;" : "=" (out) : "a" (in));// Expected error
> asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // Okay
> asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Unexpected error
> asm("foo %1,%0;" : "=," (out) : "b,b" (in)); // Okay
> asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Okay
> return out;
> }
>
> With 4.9.2, I see three errors:
>
> $ g++492 -S ex.c
> ex.c: In function 'int f(int, int)':
> ex.c:3:47: error: 'asm' operand has impossible constraints
> asm("foo %1,%0;" : "=" (out) : "b" (in));// Expected error
>^
> ex.c:4:47: error: 'asm' operand has impossible constraints
> asm("foo %1,%0;" : "=" (out) : "a" (in));// Expected error
>^
> ex.c:6:52: error: 'asm' operand has impossible constraints
> asm("foo %1,%0;" : "=," (out) : "a,a" (in)); // Unexpected error
> ^
>
> The first two are expected, but I'm at a loss to explain the third error.
> In all four of the multi alternative constrain cases, there is exactly one
> "good" constraint and one "bad" constraint in the pair, yet only one of the
> four cases is diagnosed.
>
> What is the expected behavior for this case?
Confirmed that I get errors in the same places as you; I'm not sure exactly
what the expected behavior is though. Maybe someone else can answer.
