Messages in this thread | | | Date | Tue, 14 Apr 2020 17:07:49 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64: Delete the space separator in __emit_inst |
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On Tue, Apr 14, 2020 at 08:43:07AM -0700, Fangrui Song wrote: > > On 2020-04-14, Mark Rutland wrote: > > Hi Fangrui, > > > > On Sun, Apr 12, 2020 at 08:38:11PM -0700, Fangrui Song wrote: > > > Many instances of __emit_inst(x) expand to a directive. In a few places > > > it is used as a macro argument, e.g. > > > > > > arch/arm64/include/asm/sysreg.h > > > #define __emit_inst(x) .inst (x) > > > > > > arch/arm64/include/asm/sysreg.h > > > #define SET_PSTATE_PAN(x) __emit_inst(0xd500401f | PSTATE_PAN | ((!!x) << PSTATE_Imm_shift)) > > > > > > arch/arm64/kvm/hyp/entry.S > > > ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN) > > > > > > Clang integrated assembler parses `.inst (x)` as two arguments passing > > > to a macro. We delete the space separator so that `.inst(x)` will be > > > parsed as one argument. > > > > I'm a little confused by the above; sorry if the below sounds stupid or > > pedantic, but I just want to make sure I've understood the problem > > correctly. > > > > For the above, ALTERNATIVE() and SET_PSTATE_PAN() are both preprocessor > > macros, so I would expect those to be expanded before either the > > integrated assembler or an external assembler consumes any of the > > assembly (and both would see the same expanded text). Given that, I'm a > > bit confused as to why the integrated assembly would have an impact on > > preprocessing. > > > > Does compiling the pre-processed source using the integrated assembler > > result in the same behaviour? Can we see the expanded text to make that > > clear? > > > > ... at what stage exactly does this go wrong? > > > > Thanks, > > Mark. > > Hi Mark, > > The C preprocessor expands arch/arm64/kvm/hyp/entry.S > ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN) > > to: > alternative_insn nop, .inst (0xd500401f | ((0) << 16 | (4) << 5) | ((!!1) << 8)), 4, 1 > > `alternative_insn` is an assembler macro, not handled by the C preprocessor. > > Both comma and space are separators, with an exception that content > inside a pair of parentheses/quotes is not split, so clang -cc1as or GNU > as x86 splits arguments this way: > > nop, .inst, (0xd500401f | ((0) << 16 | (4) << 5) | ((!!1) << 8)), 4, 1
Thanks for this; I understand now.
Could we fold that into the commit message? I think this is much clearer than the current wording. The explicit description of separator behaviour, the pre-expansion of the CPP macros, and the example of how the assembler will split this really help.
> I actually feel that GNU as arm64's behavior is a little bit buggy. It > works just because GNU as has another preprocessing step `do_scrub_chars` > and its arm64 backend deletes the space before '(' > > alternative_insn nop,.inst(0xd500401f|((0)<<16|(4)<<5)|((!!1)<<8)),4,1 > > The x86 backend keeps the space before the outmost '(' > > alternative_insn nop,.inst (0xd500401f|((0)<<16|(4)<<5)|((!!1)<<8)),4,1 > > By reading its state machine, I think keeping the spaces will be the > most reasonable behavior.
I think I agree. This deviation across architectures is unfortunate for such a low-level but common tool.
> If .inst were only used as arguments, > > alternative_insn nop, ".inst (0xd500401f | ((0) << 16 | (4) << 5) | ((!!1) << 8))", 4, 1 > > would be the best to avoid parsing issues. > > > > > > > Note, GNU as parsing `.inst (x)` as one argument is unintentional (for > > > example the x86 backend will parse the construct as two arguments). > > > See https://sourceware.org/bugzilla/show_bug.cgi?id=25750#c10 > > > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/939 > > > Cc: clang-built-linux@googlegroups.com > > > Signed-off-by: Fangrui Song <maskray@google.com> > > > --- > > > arch/arm64/include/asm/sysreg.h | 4 +++- > > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h > > > index ebc622432831..af21e2ec5e3e 100644 > > > --- a/arch/arm64/include/asm/sysreg.h > > > +++ b/arch/arm64/include/asm/sysreg.h > > > @@ -49,7 +49,9 @@ > > > #ifndef CONFIG_BROKEN_GAS_INST > > > > > > #ifdef __ASSEMBLY__ > > > -#define __emit_inst(x) .inst (x) > > > +// The space separator is omitted so that __emit_inst(x) can be parsed as > > > +// either a directive or a macro argument. > > > +#define __emit_inst(x) .inst(x)
Can we make this a bit more explicit and say "assembler macro argument"? That way we can avoid any confusion with a CPP macro.
With that (and with the details above folded into the commit message):
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Thanks, Mark.
> > > #else > > > #define __emit_inst(x) ".inst " __stringify((x)) "\n\t" > > > #endif > > > -- > > > 2.26.0.110.g2183baf09c-goog > > >
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