From: Jeroen Demeyer <jdemeyer@cage.ugent.be>
To: Ian Lance Taylor <iant@google.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Inline assembly constraints question
Date: Wed, 20 Aug 2008 13:47:00 -0000 [thread overview]
Message-ID: <48AC1FC6.4060308@cage.ugent.be> (raw)
In-Reply-To: <m3ej4l7xm2.fsf@google.com>
[-- Attachment #1: Type: text/plain, Size: 1565 bytes --]
Ian Lance Taylor wrote:
> Jeroen Demeyer <jdemeyer@cage.ugent.be> writes:
>
>> I have a C++ program (i386 target) which has a piece of inline
>> assembly with the following constraints:
>>
>> asm(/* Some asm code */
>> : "=&rm" (n0), "=&r" (n1), "=&r" (n2)
>> : "2" (n0), "1" (n1), "g" (n2), "cI" (s)
>> );
>>
>> When compiled with g++ 4.1.2 (CXXFLAGS=-O2 -march=pentium4) the
>> operands %0 and %5 get the same memory address, even though they refer
>> to distinct variables (n0 and n2).
>
> It would help to see the types of the variables. Is n2 a pointer?
> Also it would help to see the actual values that wind up getting
> passed in.
n0, n1, n2 are unsigned long (members of the same C++ class, the asm
statement is in a member function), s is unsigned char.
> I think more details would help. For example, run gcc with the -dg
> option, look at FILENAME.xxx.greg, and show us the insn corresponding
> to the asm statement.
This is the full asm statement:
asm("# number_t& number_t::operator>>=(unsigned char s)\n"
" shrdl %6, %4, %3\n"
" movl %3, %0\n"
" movl %5, %2\n"
" shrdl %6, %2, %1\n"
" shrl %6, %2\n"
: "=&rm" (n0), "=&r" (n1), "=&r" (n2)
: "2" (n0), "1" (n1), "g" (n2), "cI" (s)
);
The code generated is the following:
shrdl %cl, %esi, %eax
movl %eax, -4384(%ebp)
movl -4384(%ebp), %eax
shrdl %cl, %eax, %esi
shrl %cl, %eax
As you can see, %0 and %5 both become -4384(%ebp).
See attachment for the output from -dg.
I hope this clarifies the problem.
Thank you!
[-- Attachment #2: test.cpp.37.greg --]
[-- Type: text/plain, Size: 2503 bytes --]
(insn:HI 809 1339 1338 109 (parallel [
(set (mem/c:SI (plus:SI (reg/f:SI 6 bp)
(const_int -4384 [0xffffeee0])) [88 t$n0.805+0 S4 A8])
(asm_operands:SI ("# number_t& number_t::operator>>=(unsigned char s)
shrdl %6, %4, %3
movl %3, %0
movl %5, %2
shrdl %6, %2, %1
shrl %6, %2
") ("=&rm") 0 [
(reg:SI 0 ax)
(reg:SI 4 si)
(mem/c:SI (plus:SI (reg/f:SI 6 bp)
(const_int -4384 [0xffffeee0])) [88 t$n2+0 S4 A8])
(reg:QI 2 cx)
]
[
(asm_input:SI ("2"))
(asm_input:SI ("1"))
(asm_input:SI ("g"))
(asm_input:QI ("cI"))
] ("i386/number96.h") 148))
(set (reg:SI 4 si)
(asm_operands:SI ("# number_t& number_t::operator>>=(unsigned char s)
shrdl %6, %4, %3
movl %3, %0
movl %5, %2
shrdl %6, %2, %1
shrl %6, %2
") ("=&r") 1 [
(reg:SI 0 ax)
(reg:SI 4 si)
(mem/c:SI (plus:SI (reg/f:SI 6 bp)
(const_int -4384 [0xffffeee0])) [88 t$n2+0 S4 A8])
(reg:QI 2 cx)
]
[
(asm_input:SI ("2"))
(asm_input:SI ("1"))
(asm_input:SI ("g"))
(asm_input:QI ("cI"))
] ("i386/number96.h") 148))
(set (reg:SI 0 ax)
(asm_operands:SI ("# number_t& number_t::operator>>=(unsigned char s)
shrdl %6, %4, %3
movl %3, %0
movl %5, %2
shrdl %6, %2, %1
shrl %6, %2
") ("=&r") 2 [
(reg:SI 0 ax)
(reg:SI 4 si)
(mem/c:SI (plus:SI (reg/f:SI 6 bp)
(const_int -4384 [0xffffeee0])) [88 t$n2+0 S4 A8])
(reg:QI 2 cx)
]
[
(asm_input:SI ("2"))
(asm_input:SI ("1"))
(asm_input:SI ("g"))
(asm_input:QI ("cI"))
] ("i386/number96.h") 148))
(clobber (reg:QI 19 dirflag))
(clobber (reg:QI 18 fpsr))
(clobber (reg:QI 17 flags))
]) -1 (nil)
(nil))
next prev parent reply other threads:[~2008-08-20 13:45 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-08-18 10:34 Jeroen Demeyer
2008-08-19 16:59 ` Ian Lance Taylor
2008-08-20 13:47 ` Jeroen Demeyer [this message]
2008-08-20 19:31 ` Ian Lance Taylor
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