[Bug target/50304] poor code for accessing certain element of arrays

["tom at deltasystem dot hu" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50304

--- Comment #4 from Tamas Fenyvesi <tom at deltasystem dot hu> 2011-09-08 13:16:53 UTC ---
Please find a sample code and its objdump-ed asm in the attachment.

The command line is:
arm-none-eabi-gcc -D__REDLIB__ -DDEBUG -D__USE_CMSIS -D__CODE_RED -I"../cmsis"
-I"../config" -I"../driver" -O1 -g3 -Wall -c -fmessage-length=0 -fno-builtin
-ffunction-sections -fdata-sections -mcpu=cortex-m0 -mthumb -MMD -MP
-MF"src/bugreport.d" -MT"src/bugreport.d" -o"src/bugreport.o"
"../src/bugreport.c"

Some comments:
-It is the same from -O1 up to -O3. The -O0 is worse.

-Access of an int array differs somewhat but both have adding (or relative)
operation. Access of a struct doesn't differ in anything whether or not uses a
*const.

-All (* const) should have been exist (rather than have been replaced by some
adding of two (or more) other adders). It definetely costs more (in code area)
to have some offset vector and adding code than to have a precomputed ofsetted
address (even if the base address were stored elsewhere), not to mention the
huge wasted runtime. There can appear several cascaded adding operations for
computing a single address. The code is larger and much slower, plus it needs
more register to allocate, which in turn also increase code size and required
runtime.

-Why does the compiler resolve the mode of computing the ofsetted address
instead of simply materializing a *const if the user explicitly instructs it to
hace a *const? The code can have any number of copies of *const address (as it
can't change) if it were required for e.g. a pc-relative loading. There's
simply no point in not direct materializing any *const.

-Precompiling any known addesses and adding only the variables, on the other
hand, is good practice (and not uncommon in other compilers than gcc), even
without any *const. (E.g. for "a[1][2][var]" it should precompile "a[1][2]" and
add only "var".)