Re: [PATCH] x86: use gcc6'es flags asm() output support

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 01/07/16 17:10, Jan Beulich wrote:
>>>> On 01.07.16 at 17:38, <andrew.cooper3@citrix.com> wrote:
>> As for interleaving inside the asm statement itself, we already have
>> precedent for that with the HAVE_GAS_* predicates.  It would make the
>> patch rather larger, but might end up looking cleaner.  It is probably
>> also worth switching to named parameters to reduce the risk of getting
>> positional parameters out of order.
> So taking just the first example I've converted: Do you think this
>
> static bool_t even_parity(uint8_t v)
> {
>     asm ( "test %1,%1"
> #ifdef __GCC_ASM_FLAG_OUTPUTS__
>           : "=@ccp" (v)
> #else
>           "; setp %0"
>           : "=qm" (v)
> #endif
>           : "q" (v) );
>
>     return v;
> }
>
> is better than the original?

How about a different example, from the second hunk

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 460d1f7..8d52a41 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -832,8 +832,19 @@ static int read_ulong(
 static bool_t mul_dbl(unsigned long m[2])
 {
     bool_t rc;
-    asm ( "mul %1; seto %2"
-          : "+a" (m[0]), "+d" (m[1]), "=qm" (rc) );
+
+    asm ( "mul %1;"
+#ifndef __GCC_ASM_FLAG_OUTPUTS__
+          "seto %[rc];"
+#endif
+          : "+a" (m[0]), "+d" (m[1]),
+#ifdef __GCC_ASM_FLAG_OUTPUTS__
+            [rc] "=@cco" (rc)
+#else
+            [rc] "=qm" (rc)
+#endif
+        );
+
     return rc;
 }
 
This at least doesn't mix the : inside an #ifdef

> I'm unsure, and I'm actually inclined to
> think that then the abstraction alternative might look better.

If the abstraction comes in two parts, one which may insert a `setcc`
instruction, and one which selects between =qm and =@cc, it wouldn't end
up hiding the :.

~Andrew

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