To: the helpful people at gcc-help at gcc.gnu.org:
In an embedded application running on an AMCC PowerPC PPC405EP
CPU, I expected that there would be no READS or WRITES into
the true ".text" segment using GCC-4.2.0, but I was wrong.
(I expected it to be execute- or fetch-only conceptually.)
Here is my GCC version information:
[~]$ /usr/local/bin/powerpc-elf-gcc --ver
Using built-in specs.
Target: powerpc-elf
Configured with: ../gcc-4.2.0/configure --prefix=/usr/local/trpz/trpz4.2
--target=powerpc-elf --enable-64-bit-bfd --disable-libssp
Thread model: single
gcc version 4.2.0
Depicted below is an excerpt from the objdump output
of one of the PPC ELF files. We found 78 cases of this
situation in the ELF file (which includes 3917 functions).
Each of the 78 cases have an identical look to them,
which is essentially this:
(1) a function ends with its "blr"
(2) the next .text word shows like: .long 0x1579c4
(3) the entry of the next function uses r30 to do a
read into the .text segment of a pointer which
points into the .rodata segment
The questions I have are:
(#1):
Why does GCC store certain pointers in the .text segment,
rather than in the .rodata or .data segments? How does it
choose which pointers deserve to reside in the .text seg?
(#2):
What are these pointers to .rodata officially called?
(#3):
Is there a GCC option that might be used to force
these pointers out of the .text segment?
We were using PPC debug control registers to trigger
on any application software READS or WRITES into the
.text segment, to look for wild-reads or wild-writes,
but this GCC construct -- whatever it's named -- is
defeating the DBCR-read trigger: we get constant
interrupts from the 78-instances noted.
THANKS for any direction on this topic.
I tried to search for things like:
GCC pointers stored in text segment
... but got nowhere. Most group discussions were about
string-constants in .rodata which were still writeable.
Many thanks!
-- Jim R.
SAMPLE_EXCERPT_FROM_OBJDUMP_OF_ELF_FILE:
/data/me/cvs/main/common/util/patricia.c:1141
}
}
}
}
c2f84: 7f e3 fb 78 mr r3,r31
c2f88: 80 01 00 44 lwz r0,68(r1)
c2f8c: 7c 08 03 a6 mtlr r0
c2f90: ba 41 00 08 lmw r18,8(r1)
c2f94: 38 21 00 40 addi r1,r1,64
c2f98: 4e 80 00 20 blr
c2f9c: 00 15 79 c4 .long 0x1579c4
000c2fa0 <n_doprnt>:
n_doprnt():
static int
n_doprnt(char *obuf, unsigned int obufsize, const char *fmt0, va_list argp)
{
c2fa0: 94 21 ff 00 stwu r1,-256(r1)
c2fa4: 7c 08 02 a6 mflr r0
c2fa8: bd c1 00 b8 stmw r14,184(r1)
c2fac: 90 01 01 04 stw r0,260(r1)
c2fb0: 42 9f 00 05 bcl- 20,4*cr7+so,c2fb4 <n_doprnt+0x14>
c2fb4: 7f c8 02 a6 mflr r30
c2fb8: 80 1e ff e8 lwz r0,-24(r30)
c2fbc: 7f c0 f2 14 add r30,r0,r30
c2fc0: 90 61 00 a0 stw r3,160(r1)
c2fc4: 7c d2 33 78 mr r18,r6
/data/me/cvs/main/common/util/printf.c:98
uchar *fmt; /* format string */
int ch; /* character from fmt */
int cnt; /* return value accumulator */
int n; /* random handy integer */
char *t; /* buffer pointer */
ulong _ulong = 0; /* integer arguments %[diouxX] */
unsigned long long int _ulonglong = 0; /* long long integer
arguments %[diouxX] */
int base; /* base for [diouxX] conversion */
int dprec; /* decimal precision in [diouxX] */
int fieldsz; /* field size expanded by sign, etc */
int flags; /* flags as above */
int prec; /* precision from format (%.3d), or -1 */
int realsz; /* field size expanded by decimal precision */
int size; /* size of converted field or string */
int width; /* width from format (%8d), or 0 */
char sign; /* sign prefix (' ', '+', '-', or \0) */
char *digs; /* digits for [diouxX] conversion */
char buf[BUF+1]; /* space for %c, %[diouxX], %[eEfgG] */
char * bufstart = obuf; /* remember the start of the buffer */
if (obufsize == 0) {
c2fc8: 39 20 00 00 li r9,0
c2fcc: 91 21 00 9c stw r9,156(r1)
c2fd0: 7c 9b 23 79 mr. r27,r4
c2fd4: 41 82 0c a0 beq- c3c74 <n_doprnt+0xcd4>
/data/me/cvs/main/common/util/printf.c:102
return 0;
}
fmt = (uchar *)fmt0;
c2fd8: 7c b6 2b 78 mr r22,r5
c2fdc: 83 41 00 a0 lwz r26,160(r1)
c2fe0: 39 40 00 00 li r10,0
c2fe4: 91 41 00 9c stw r10,156(r1)
c2fe8: 3a 20 00 00 li r17,0
c2fec: 3a 80 00 00 li r20,0
c2ff0: 3a a0 00 00 li r21,0
c2ff4: 81 fe 80 00 lwz r15,-32768(r30)
/data/me/cvs/main/common/util/printf.c:367
digs = "0123456789abcdef";
