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Date: Sun, 23 Sep 2018 02:11:42 +0000
From: Pascal Cuoq <cuoq@...st-in-soft.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Re: un-UBify-strings
Hello Rich,
On 23 Sep 2018, at 02:35, Rich Felker <dalias@...c.org<mailto:dalias@...c.org>> wrote:
I've had this patch sitting around since 2016, and just updated it to
apply cleanly. Any objections?
Your patch contains:
...
size_t __attribute__((__may_alias__)) *wd;
const size_t __attribute__((__may_alias__)) *ws;
...
In my experience, this use of __may_alias__ does not do anything. See function f in the example below, which both GCC and Clang optimize as if the programmer had not used __may_alias__ at all: https://gcc.godbolt.org/z/Um4NU7
You should use a typdef for the aliasing type, as shown for function g (in with GCC and Clang do not apply the optimization).
The example in GCC's documentation for __may_alias__ also uses a typedef: https://gcc.gnu.org/onlinedocs/gcc-4.0.4/gcc/Type-Attributes.html
Pascal
__________________________
#include<string.h>
doubleX,Y;
voidf(void*d, constvoid*s) {
size_t __attribute__((__may_alias__)) *wd;
wd = d;
X = 1.0;
*wd = 1;
Y = X;
}
typedefsize_t __attribute__((__may_alias__)) aliasing_word;
voidg(void*d, constvoid*s) {
aliasing_word *wd;
wd = d;
X = 1.0;
*wd = 1;
Y = X;
}
Clang 6.0.0 -O3 -fomit-frame-pointer:
f:# @f
movabsq$4607182418800017408, %rax# imm = 0x3FF0000000000000
movq%rax, X(%rip)
movq$1, (%rdi)
movq%rax, Y(%rip)
retq
g:# @g
movabsq$4607182418800017408, %rax# imm = 0x3FF0000000000000
movq%rax, X(%rip)
movq$1, (%rdi)
movqX(%rip), %rax
movq%rax, Y(%rip)
retq
GCC 8.2 -O3 -fomit-frame-pointer:
f:
movsd.LC0(%rip), %xmm0
movsd%xmm0, X(%rip)
movq$1, (%rdi)
movsd%xmm0, Y(%rip)
ret
g:
movq.LC0(%rip), %rax
movq%rax, X(%rip)
movq$1, (%rdi)
movsdX(%rip), %xmm0
movsd%xmm0, Y(%rip)
ret
.LC0:
.long0
.long1072693248
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