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Re: VIRT_ADDR_VARIES
From: |
Paul Eggert |
Subject: |
Re: VIRT_ADDR_VARIES |
Date: |
Thu, 10 Nov 2011 08:19:44 -0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
On 11/10/11 01:29, Andreas Schwab wrote:
>> + ((uintptr_t) XPNTR (obj) - (uintptr_t) pure <= PURESIZE)
>
> There is no need for obfuscation, the compiler should find that out by
> itself.
Perhaps it should, but it typically doesn't. For example, given this:
#include <inttypes.h>
#include <stddef.h>
#define TYPEMASK 7
#define XPNTR(a) ((a) & ~TYPEMASK)
#define PURESIZE 1000000
extern long pure[];
#define PURE_P1(obj) \
((char *) pure <= (char *) XPNTR (obj) \
&& (char *) XPNTR (obj) < (char *) pure + PURESIZE)
#define PURE_P2(obj) \
((uintptr_t) XPNTR (obj) - (uintptr_t) pure <= PURESIZE)
int pure1 (intptr_t obj) { return PURE_P1 (obj); }
int pure2 (intptr_t obj) { return PURE_P2 (obj); }
gcc -O2 (x86-64, GCC 4.6.2) generates this for the comparisons of PURE_P1:
xorl %eax, %eax
cmpq $pure, %rdi
jb .L2
xorl %eax, %eax
cmpq $pure+1000000, %rdi
setb %al
.L2: rep
and this for PURE_P2:
xorl %eax, %eax
subq $pure, %rdi
cmpq $1000000, %rdi
setbe %al
The PURE_P1 version has more instructions, and has a conditional jump.
The PURE_P2 version is nearly as fast as the old code without
VIRT_ADDR_VARIES. I observed similar savings with the other
compilers I tried (Sun C 5.11 sparcv9 cc -xO4, clang 2.8 x86-64 -O2).
So we should go with PURE_P2.
- Re: VIRT_ADDR_VARIES, (continued)
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/07
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/08
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/08
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/08
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/08
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/08
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/09
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/09
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/10
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/10
- Re: VIRT_ADDR_VARIES,
Paul Eggert <=
- Re: VIRT_ADDR_VARIES, Eli Zaretskii, 2011/11/10
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/10
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/10
- Re: VIRT_ADDR_VARIES, Eli Zaretskii, 2011/11/10
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/10
- Re: VIRT_ADDR_VARIES, Eli Zaretskii, 2011/11/10
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/13
- Re: VIRT_ADDR_VARIES, Stephen J. Turnbull, 2011/11/09
- Re: VIRT_ADDR_VARIES, Paul Eggert, 2011/11/10
- Re: VIRT_ADDR_VARIES, Andreas Schwab, 2011/11/10