https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65640
Eric Gallager <egallager at gcc dot gnu.org> 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 <egallager at gcc dot gnu.org> --- (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;" : "=&a" (out) : "b" (in)); // Okay > asm("foo %1,%0;" : "=&b" (out) : "b" (in)); // Expected error > asm("foo %1,%0;" : "=&a" (out) : "a" (in)); // Expected error > asm("foo %1,%0;" : "=&a,&b" (out) : "b,b" (in)); // Okay > asm("foo %1,%0;" : "=&a,&b" (out) : "a,a" (in)); // Unexpected error > asm("foo %1,%0;" : "=&b,&a" (out) : "b,b" (in)); // Okay > asm("foo %1,%0;" : "=&b,&a" (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;" : "=&b" (out) : "b" (in)); // Expected error > ^ > ex.c:4:47: error: 'asm' operand has impossible constraints > asm("foo %1,%0;" : "=&a" (out) : "a" (in)); // Expected error > ^ > ex.c:6:52: error: 'asm' operand has impossible constraints > asm("foo %1,%0;" : "=&a,&b" (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.