On 1 July 2017 at 23:35, Richard Henderson <r...@twiddle.net> wrote: > On 07/01/2017 03:30 PM, Peter Maydell wrote: >> >> On 1 July 2017 at 23:20, Richard Henderson <r...@twiddle.net> wrote: >>> That is an absolutely stupid warning. There's long precedent for the >>> compiler choosing the prefix for you based on the type of the argument. >> >> >> Isn't that the problem? The type of the argument says "32 bits" >> but the instruction here really wants 64 bits (MRS takes Xn, not Wn). > > > The warning is telling me to use %w to force Wn. So if the assembler really > doesn't like Wn, the warning is a bit more than confusing.
Wouldn't be the first time a compiler has produced a confusing warning :-) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63359 includes some previous gcc-vs-clang-dev discussion on the topic of the warning. It looks like the clang dev rationale is that having %0 always generate a 64-bit register access even when passed a 32-bit value is confusing (eg people expect "str %0, [addr]" : ... : "r" (var_32bits)" to do a 32 bit store, not a 64 bit store), so better to warn and nudge the code author into being explicit about the size they wanted. > Perhaps it ought to be telling me to use %x to force Xn in spite of the > type? You always get Xn anyway, regardless of the type. For us, I think the right thing to do is make 'ctr' be a uint64_t, because we're reading a 64 bit sysreg and silently truncating it as a side effect of the asm constraints is a bit obscure. thanks -- PMM
