i386: gcc & asm(): wrong constraint for "mull"
Hello,
I noticed (with some inspiration from Andy Kleen) that some asm() instructions for the ia32 use the "g" constraint for "mull", where my Intel 386 Assembly Language Manual suggests the "MUL" instruction needs an r/m operand. So I guess the correct constraint is "rm" in gcc, and not "g". That change identical assembly output for gcc-2.95.2, but some gcc-2.96.x will try a multiplication with an immediate (constant) operand for the "g" constarint, and the as will choke on that. (Redhat 7.0 ships such a version of gcc).
As I won't be online next week, let me say
regards and a good new year to all!
Ulrich
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected]
Please read the FAQ at http://www.tux.org/lkml/
Quoted 10 lines · Ulrich Windl
Hello,
I noticed (with some inspiration from Andy Kleen) that some
asm()instructions for the ia32 use the "g" constraint for "mull", where my Intel 386 Assembly Language Manual suggests the "MUL" instruction needs an r/m operand. So I guess the correct constraint is "rm" in gcc, and not "g". That change identical assembly output for gcc-2.95.2, but some gcc-2.96.x will try a multiplication with an immediate (constant) operand for the "g" constarint, and the as will choke on that. (Redhat 7.0 ships such a version of gcc).
gcc 2.95.2 md.texi sais:
@cindex @samp{g} in constraint
@item @samp{g}
Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.
(2.95.2 was chosen to make it clear it is not something new in gcc). That means gcc is really free to choose which of register, memory or immediate it puts in and the fact that some gcc version choose one and others choose other is perfectly correct.
Fix the constraints and be happy (at least during the upcoming millenium) :)
Jakub
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected]
Please read the FAQ at http://www.tux.org/lkml/
Quoted 18 lines · Jakub Jelinek
Quoted 10 lines · Ulrich Windl
Hello,
I noticed (with some inspiration from Andy Kleen) that some
asm()instructions for the ia32 use the "g" constraint for "mull", where my Intel 386 Assembly Language Manual suggests the "MUL" instruction needs an r/m operand. So I guess the correct constraint is "rm" in gcc, and not "g". That change identical assembly output for gcc-2.95.2, but some gcc-2.96.x will try a multiplication with an immediate (constant) operand for the "g" constarint, and the as will choke on that. (Redhat 7.0 ships such a version of gcc).gcc 2.95.2 md.texi sais:
@cindex @samp{g} in constraint
@item @samp{g}
Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.
Quoted 5 lines
(2.95.2 was chosen to make it clear it is not something new in gcc). That means gcc is really free to choose which of register, memory or immediate it puts in and the fact that some gcc version choose one and others choose other is perfectly correct.
Fix the constraints and be happy (at least during the upcoming millenium) :)
Oh, if it wasn't clear: It's what I wanted to say. As I don't have a patch ready for that, maybe start at arch/i386/kernel/time.c; there are at least two of these "mull" instructions.
Ulrich
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected]
Please read the FAQ at http://www.tux.org/lkml/