-----Original Message-----
From: Hofman, Gertjan (BI35)
Sent: Friday, August 24, 2007 5:13 PM
To: 'gcc-help@xxxxxxxxxxx'
Subject: Simple Question on CPU specific Optimization Options
Dear GCC user,
I am hoping this is a simple how-to question.
I am programming an embedded at91sam9260 ATMEL ARM-920 processor, cross
compiling on Linux-i386. My program read/writes directly to memory after
getting pointers from the /dev/mem driver. i.e. using statements like:
ctrl_reg= *((unsigned char *) virtaddr); \\ a read
*((unsigned char *) virtaddr) = 0xF0; \\ a write
I have noticed that any optimization flag (-0 and up) removes
consequtive read/write lines i.e.
*((unsigned char *) virtaddr) = 0xF0;
*((unsigned char *) virtaddr) = 0xFE;
Even though they are writing different data to memory. Kind of scary.
With -O0, the code runs as anticipated.
In order to find out what is causing the removal I replaced -O with all
the -fxxxxx options listed in man gcc. This does not replicate the
problem ! In otherwords, -O does more that the sum of all the -fxxx
options.
1. Is there a way to have the compiler tell me which options are being
turned on using -O ?
2. Are there architecture specific optimizations which are not described
in the man page ?
Much appreciated
Gertjan
Gertjan Hofman, PhD
Principal Research Scientist
Honeywell Process Solutions
QCS & Sensors
Phone: 604.982.3574 (office) 604.980.3421 ext 4514 (lab)
Fax: 604.980.0120
500 Brooksbank Ave
North Vancouver, BC V7J 3S4
Canada
