Re: [PATCH][AArch64] Set SLOW_BYTE_ACCESS

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

ping




From: Wilco Dijkstra
Sent: 17 November 2017 15:21
To: GCC Patches
Cc: nd
Subject: [PATCH][AArch64] Set SLOW_BYTE_ACCESS
  

Contrary to all documentation, SLOW_BYTE_ACCESS simply means accessing
bitfields by their declared type, which results in better codegeneration on practically
any target.

I'm thinking we should completely remove all trace of SLOW_BYTE_ACCESS
from GCC as it's confusing and useless.

OK for commit until we get rid of it?

ChangeLog:
2017-11-17  Wilco Dijkstra  <wdijkstr@arm.com>

    gcc/
        * config/aarch64/aarch64.h (SLOW_BYTE_ACCESS): Set to 1.
--
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 056110afb228fb919e837c04aa5e5552a4868ec3..d8f4d129a02fb89eb00d256aba8c4764d6026078 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -769,14 +769,9 @@ typedef struct
    if given data not on the nominal alignment.  */
 #define STRICT_ALIGNMENT                TARGET_STRICT_ALIGN
 
-/* Define this macro to be non-zero if accessing less than a word of
-   memory is no faster than accessing a word of memory, i.e., if such
-   accesses require more than one instruction or if there is no
-   difference in cost.
-   Although there's no difference in instruction count or cycles,
-   in AArch64 we don't want to expand to a sub-word to a 64-bit access
-   if we don't have to, for power-saving reasons.  */
-#define SLOW_BYTE_ACCESS               0
+/* Contrary to all documentation, this enables wide bitfield accesses,
+   which results in better code when accessing multiple bitfields.  */
+#define SLOW_BYTE_ACCESS               1
 
 #define NO_FUNCTION_CSE 1