linux-kernel - Re: [PATCH v2 4/6] compiler-gcc.h: add asm

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Date:   Thu, 5 Sep 2019 13:07:11 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        "gcc-patches@....gnu.org" <gcc-patches@....gnu.org>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

On 05/09/2019 02.18, Nick Desaulniers wrote:
> On Fri, Aug 30, 2019 at 4:15 PM Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>>
>> This adds an asm_inline macro which expands to "asm inline" [1] when gcc
>> is new enough (>= 9.1), and just asm for older gccs and other
>> compilers.
>>
>> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
>> ---
>>  include/linux/compiler-gcc.h   | 4 ++++
>>  include/linux/compiler_types.h | 4 ++++
>>  2 files changed, 8 insertions(+)
>>
>> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>> index d7ee4c6bad48..544b87b41b58 100644
>> --- a/include/linux/compiler-gcc.h
>> +++ b/include/linux/compiler-gcc.h
>> @@ -172,3 +172,7 @@
>>  #endif
>>
>>  #define __no_fgcse __attribute__((optimize("-fno-gcse")))
>> +
>> +#if GCC_VERSION >= 90100
> 
> Is it too late to ask for a feature test macro? Maybe one already
> exists? 

No, not as far as I know. Perhaps something like below, though that
won't affect the already released gcc 9.1 and 9.2, of course.

gcc maintainers, WDYT? Can we add a feature test macro for asm inline()?
For context, I'm trying to add an asm_inline macro to the kernel source
that will fall back to asm when "asm inline" is not supported - see
https://lore.kernel.org/lkml/20190830231527.22304-1-linux@rasmusvillemoes.dk/
for the whole thread.

From: Rasmus Villemoes <rv@...musvillemoes.dk>
Subject: [PATCH] add feature test macro for "asm inline"

Allow users to check availability of "asm inline()" via a feature test
macro. If and when clang implements support for "asm inline()", it's
easier for users if they can just test __HAVE_ASM_INLINE rather than
juggling different version checks for different compilers.

Changelog:

gcc/c-family/

	* c-cppbuiltin.c (c_cpp_builtins): Add pre-define for
	__HAVE_ASM_INLINE.

gcc/

	* doc/cpp.texi: Document predefine __HAVE_ASM_INLINE.
---
 gcc/c-family/c-cppbuiltin.c | 3 +++
 gcc/doc/cpp.texi            | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/gcc/c-family/c-cppbuiltin.c b/gcc/c-family/c-cppbuiltin.c
index fc68bc4d0c4..163f3058741 100644
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -1383,6 +1383,9 @@ c_cpp_builtins (cpp_reader *pfile)
   if (targetm.have_speculation_safe_value (false))
     cpp_define (pfile, "__HAVE_SPECULATION_SAFE_VALUE");

+  /* Show the availability of "asm inline()".  */
+  cpp_define (pfile, "__HAVE_ASM_INLINE");
+
 #ifdef DWARF2_UNWIND_INFO
   if (dwarf2out_do_cfi_asm ())
     cpp_define (pfile, "__GCC_HAVE_DWARF2_CFI_ASM");
diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index e271f5180d8..98f6d625857 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -2386,6 +2386,11 @@ and swap operations on operands 1, 2, 4, 8 or 16
bytes in length, respectively.
 This macro is defined with the value 1 to show that this version of GCC
 supports @code{__builtin_speculation_safe_value}.

+@...m __HAVE_ASM_INLINE
+This macro is defined with the value 1 to show that this version of GCC
+supports @code{asm inline()}.  @xref{Size of an asm,,, gcc, Using
+the GNU Compiler Collection (GCC)}.
+
 @item __GCC_HAVE_DWARF2_CFI_ASM
 This macro is defined when the compiler is emitting DWARF CFI directives
 to the assembler.  When this is defined, it is possible to emit those same