Re: #define SLOW_BYTE_ACCESS in i386.h

["H. J. Lu" <hjl at lucon dot org>]

On Fri, Sep 01, 2006 at 11:03:31AM -0700, Eric Christopher wrote:
> Hui-May Chang wrote:
> >I have a question regarding "#define SLOW_BYTE_ACCESS" in i386.h. It is 
> >used in "get_best_mode" routine which finds the best mode to use when 
> >referencing a bit field. It is currently set to 0. If it is set it to 1, 
> >it means "accessing less than a word of memory is no faster than 
> >accessing a word of memory".  I experimented with it and observed great 
> >performance improvement. It is set to 1 for some other configurations 
> >(e.g., rs6000, pa, ia64). Is it always a win to set it? Is it better to 
> >set it for certain i386 architectures?
> >
> 
> I'll bet that it's probably advantageous to set it for a couple of 
> reasons in the new chips at least:
> 
> 1) You avoid the problem that got you here of large bitfields needing 
> shift/insert operations
> 
> 2) You avoid length changing since you're mostly operating on things in 
> word mode.
> 
> However, I'm not an expert on the chip so I'd suggest posting a small 
> testcase that shows #1 for people and the resultant code differences so 
> they can see the difference. Hopefully someone with more intel 
> experience (like HJ or Jan) can comment on whether or not this is a good 
> general idea for the processor.
> 

I tried this patch and enabled it for Conroe and Nocona. It doesn't
have much impact on SPEC CPU 2000 on Conroe and it seems bad on Nocona.
Maybe we should add -mslow-byte-access to investigate it further.


H.J.
---
--- gcc/config/i386/i386.c.slow	2006-08-23 17:15:14.000000000 -0700
+++ gcc/config/i386/i386.c	2006-08-24 12:10:25.000000000 -0700
@@ -831,6 +831,8 @@ const int x86_cmpxchg16b = m_NOCONA;
 const int x86_xadd = ~m_386;
 const int x86_pad_returns = m_ATHLON_K8 | m_GENERIC;
 
+const int x86_slow_byte_access = 0;
+
 /* In case the average insn count for single function invocation is
    lower than this constant, emit fast (but longer) prologue and
    epilogue code.  */
--- gcc/config/i386/i386.h.slow	2006-08-23 17:15:14.000000000 -0700
+++ gcc/config/i386/i386.h	2006-08-24 12:01:49.000000000 -0700
@@ -164,6 +164,7 @@ extern const int x86_use_bt;
 extern const int x86_cmpxchg, x86_cmpxchg8b, x86_cmpxchg16b, x86_xadd;
 extern const int x86_use_incdec;
 extern const int x86_pad_returns;
+extern const int x86_slow_byte_access;
 extern int x86_prefetch_sse;
 
 #define TARGET_USE_LEAVE (x86_use_leave & TUNEMASK)
@@ -219,6 +220,7 @@ extern int x86_prefetch_sse;
 #define TARGET_USE_INCDEC (x86_use_incdec & TUNEMASK)
 #define TARGET_PAD_RETURNS (x86_pad_returns & TUNEMASK)
 #define TARGET_EXT_80387_CONSTANTS (x86_ext_80387_constants & TUNEMASK)
+#define TARGET_SLOW_BYTE_ACCESS (x86_slow_byte_access & TUNEMASK)
 
 #define ASSEMBLER_DIALECT (ix86_asm_dialect)
 
@@ -1840,7 +1842,7 @@ do {							\
    subsequent accesses occur to other fields in the same word of the
    structure, but to different bytes.  */
 
-#define SLOW_BYTE_ACCESS 0
+#define SLOW_BYTE_ACCESS TARGET_SLOW_BYTE_ACCESS
 
 /* Nonzero if access to memory by shorts is slow and undesirable.  */
 #define SLOW_SHORT_ACCESS 0