> 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
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