AW: 68000 issue in longlong.h

[selco at t-online.de]

>  With his change the tests pass so I think the change is reasonable.

>I disagree.  If I am right that the inline asm is correct, changing it
>to avoid triggering a compiler bug is not a reasonable change.  But
>perhaps I am wrong and the inline asm is buggy, but then somebody needs
>to explain in what way it is buggy.
 
>Torbjörn


Of course I do not want to hide a compiler error here and disturb other platforms. If this turns out to be a compiler error that would be a nice finding as well!


I looked in https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

>6.47.2.6 Clobbers and Scratch Registers
>....
>Rather than allocating fixed registers via clobbers to provide scratch registers for an asm statement, an alternative is to define a variable and make it an early-clobber output.

I think that is done in the inline assembler code with the variables 
USItype __umul_tmp1, __umul_tmp2;

To make them an "early clobber output" the & sign has to be written before the d as it has been done for __umul_tmp2.

>6.47.2.3 Output Operands
>Use the ‘&’ constraint modifier (see Modifiers) on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated >input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction.

If you look at the nice compiler explorer at 
https://franke.ms/cex/z/oG53bK
you see the code generated for -O1 and -O0. If you add the % sign in the left window you will imediatelly see the changed code in the middle window.

Can you explain why __umul_tmp1 should not be an "early-clobber output"?

Btw,
Could you tell me what the "%2" at the input parameters line means? I did not find that in the Extended-Asm.html document.

: "%2" ((USItype)(a)), "d" ((USItype)(b)));

Kind regards Alexander