PATCH: Remove SLOW_BYTE_ACCESS from get_best_mode

[H. J. Lu]

On Fri, Sep 08, 2006 at 11:52:51PM -0700, H. J. Lu wrote:

> Why do we use SLOW_BYTE_ACCESS to control bit field access? Bit field
> currently is implemented as unsigned int, at least for x86. Accessing
> bit field as 32bit is always faster unless 4 byte accesses are
> much faster one 32bit access, which is very unlikely will be true for
> x86 in any circumstances. Is this a leftover from the old bit field
> implementation?
> 

I don't understand why we use byte instruction to access 32bit field.
There may be some hardwares where 4 byte accesses are faster than one
32bit access. But I am not aware any x86 one. I tried this patch on
Prescott, Nocona, Core and Core 2. There are no regressions in gcc
testsuites nor negative performance impact on SPEC CPU 2K.


H.J.
-----
2006-09-10  H.J. Lu  <hongjiu.lu@intel.com>

	* config/i386/i386.h (FAST_BYTE_ACCESS_BITFIELD): New.

	* stor-layout.c (FAST_BYTE_ACCESS_BITFIELD): New. Defined with
	SLOW_BYTE_ACCESS if not defined.
	(get_best_mode): Check FAST_BYTE_ACCESS_BITFIELD instead of
	SLOW_BYTE_ACCESS.

--- gcc/config/i386/i386.h.slow	2006-09-08 17:55:37.000000000 -0700
+++ gcc/config/i386/i386.h	2006-09-09 06:44:55.000000000 -0700
@@ -1844,6 +1844,8 @@ do {							\
 
 #define SLOW_BYTE_ACCESS 0
 
+#define FAST_BYTE_ACCESS_BITFIELD 0
+
 /* Nonzero if access to memory by shorts is slow and undesirable.  */
 #define SLOW_SHORT_ACCESS 0
 
--- gcc/stor-layout.c.slow	2006-09-07 11:13:09.000000000 -0700
+++ gcc/stor-layout.c	2006-09-09 06:48:07.000000000 -0700
@@ -2113,16 +2113,20 @@ fixup_unsigned_type (tree type)
 
    If no mode meets all these conditions, we return VOIDmode.
 
-   If VOLATILEP is false and SLOW_BYTE_ACCESS is false, we return the
+   If VOLATILEP is false and FAST_BYTE_ACCESS_BITFIELD is true, we return the
    smallest mode meeting these conditions.
 
-   If VOLATILEP is false and SLOW_BYTE_ACCESS is true, we return the
+   If VOLATILEP is false and FAST_BYTE_ACCESS_BITFIELD is false, we return the
    largest mode (but a mode no wider than UNITS_PER_WORD) that meets
    all the conditions.
 
    If VOLATILEP is true the narrow_volatile_bitfields target hook is used to
    decide which of the above modes should be used.  */
 
+#ifndef FAST_BYTE_ACCESS_BITFIELD
+#define FAST_BYTE_ACCESS_BITFIELD (!SLOW_BYTE_ACCESS)
+#endif
+
 enum machine_mode
 get_best_mode (int bitsize, int bitpos, unsigned int align,
 	       enum machine_mode largest_mode, int volatilep)
@@ -2151,7 +2155,7 @@ get_best_mode (int bitsize, int bitpos, 
       || (largest_mode != VOIDmode && unit > GET_MODE_BITSIZE (largest_mode)))
     return VOIDmode;
 
-  if ((SLOW_BYTE_ACCESS && ! volatilep)
+  if ((! FAST_BYTE_ACCESS_BITFIELD && ! volatilep)
       || (volatilep && !targetm.narrow_volatile_bitfield()))
     {
       enum machine_mode wide_mode = VOIDmode, tmode;