On 4/17/20 3:59 PM, Joseph Myers wrote:
> On Wed, 8 Apr 2020, Vineet Gupta via Libc-alpha wrote:
>
>> +# define _FPU_SETS(cw) __asm__ volatile ("bset %0, %0, 31 \r\n" \
>> + "sr %0, [0x301] \r\n" \
>> + : : "r" (cw))
>
> This asm doesn't look safe; it's modifying an input operand. I think the
> compiler will assume the register it puts %0 in is unmodified by the asm,
> because it's listed as an input.
Indeed this is a bug, as wiggling bit 31 was not present in inotial code.
I changed it to "+r"
> As an architecture-specific interface, it's not very clear if the
> interface for _FPU_SETS should be that it modifies the variable passed as
> an argument, but I'd guess not.
But does it not depend on whether the value is used after the fact. And with +r we
enable the compiler to DTRT.
> My suggestion would be to define the
> macro (using do { ... } while (0)) to copy the argument to a temporary
> variable, and do the bit-set operation in C code on that temporary
> variable rather than as part of the asm.
I can certainly do that if you think so.
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