4

I have a project in which I try to build a Firmware for a micro controller and try to get better control of the optimization flags used. I would like, instead of using the -O<number> flag specify the different optimization flags individually. Unfortunately there seems to be some optimization magic happening with the -O flag that I can not reproduce with individual optimization flags and I don't understand why.

Here is what I try and what is not working:

I know I can compile the Project with -O1. So I used the -Q and --help flags to output the Flags which are active when I activate the -O1 flag. I use this information to specify the different flags manually in my build process and compiling works fine but in the linking stage it fails because the .bss section does not fit into my RAM anymore (I only have 384 kByte available).

When I increase the RAM size in my linker script, linking works fine but the end of .bss section is placed at 416 kByte and the binary image is 75 % bigger then when using -O1 directly.

When I compare the flags and parameters reported by gcc, there is no difference between the two builds but the one without -O1 is still much bigger.

According to the GCC documentation (GCC Manual) does the -O flag only activate specific optimization flags, therefore it should be possible to do this manually as well (or not?)

Here are my gcc commands:

GCC call with single optimization flags

gcc -std=c99 -msoft-float -fno-inline -fdata-sections -ffunction-sections -Wall -Wextra\
-faggressive-loop-optimizations -fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments\
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdelete-null-pointer-checks\
-fdse -fearly-inlining -ffast-math -fforward-propagate -ffp-contract=fast -ffp-int-builtin-inexact\
-ffunction-cse -fgcse-lm -fguess-branch-probability -fhandle-exceptions -fif-conversion -fif-conversion2\
-finline-atomics -finline-functions-called-once -fipa-profile -fipa-pure-const -fipa-reference\
-fira-algorithm=CB -fira-hoist-pressure -fira-share-save-slots -fira-share-spill-slots -fivopts\
-fjump-tables -flifetime-dse -flifetime-dse=2 -fmath-errno -fmove-loop-invariants -fomit-frame-pointer\
-fpeephole -fplt -fprefetch-loop-arrays -fprintf-return-value -frename-registers -freorder-blocks
-frtti -fsched-critical-path-heuristic -fsched-dep-count-heuristic -fsched-group-heuristic\
-fsched-interblock -fsched-last-insn-heuristic -fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic\
-fsched-stalled-insns-dep -fschedule-fusion -fshort-enums -fshrink-wrap -fshrink-wrap-separate\
-fsigned-zeros -fsplit-ivs-in-unroller -fsplit-wide-types -fssa-backprop -fssa-phiopt -fstack-reuse=all\
-fstdarg-opt -fstrict-volatile-bitfields -fno-threadsafe-statics -ftrapping-math -ftree-bit-ccp\
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-cselim\ 
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert -ftree-loop-im\
-ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops=1 -ftree-phiprop -ftree-pta\
-ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-ter -fvar-tracking -fvar-tracking-assignments\
-fweb -fmerge-constants -fno-associative-math -fno-cx-limited-range -fno-exceptions -fno-finite-math-only\
-fno-reciprocal-math -fno-unsafe-math-optimizations -fexcess-precision=standard -qbsp=leon2 -DCPU_FREQ=CPU_FREQ_125MHz\
-fno-builtin-strtok -c -o timer.o timer.c

GCC with -O1

gcc -O1 -std=c99 -msoft-float -qbsp=leon2 -DCPU_FREQ=CPU_FREQ_125MHz -fno-builtin-strtok -c -o timer.o timer.c

If needed I could also provide the output from gcc to see what flags are active in either case. the only difference I found was, that -fexcess-precision is set to "default" with -O1. I tried both possiblities (fast and standard) but this does not make any difference.

Does anyone know what magic the -O option activates additionally which I overlooked?

Improve this question
5
  • 1
    -O activates optimizations that cannot be specified with flags. You can disable or enable optimizations that have flags. Some optimizations cannot be disabled. Commented Jan 24, 2020 at 15:35
  • What microcontroller is it? Different vendors do different weird things in their "gcc compliant" compiler. Commented Jan 24, 2020 at 16:12
  • @S.S.Anne Thank you for this information. Do you know if there is any way to find out what passes the compiler does with only the -O flag? I also just found this in the manual linked above "Most optimizations are completely disabled [...] if an -O level is not set on the command line, even if individual optimization flags are specified." This would explain this behavoir. Commented Jan 25, 2020 at 22:41
  • @yhyrcanus I use a Leon2 microcontroller which has a Sparc V8 architecture. I use the compiler provided by gaisler Commented Jan 25, 2020 at 22:43
  • 1
    Well, yeah, that too. I can't think of it off the top if my head but you can browse the GCC source code (Google it) to find out. Commented Jan 25, 2020 at 23:12

1 Answer 1

Reset to default
4

According to GCC manual

Most optimizations are only enabled if an -O level is set on the command line.
Otherwise they are disabled, even if individual optimization flags are specified.

so specifying optimization flags alone will not be enough. For example here you can see that certain analysis is enabled only if both -O and -fweb are enabled:

class pass_web : public rtl_opt_pass
{
  ...
  virtual bool gate (function *) { return (optimize > 0 && flag_web); }

Even specifying -O1 and selectively enabling optimizations from higher optimization levels will not work reliably because some passes explicitly rely on -O value. E.g. here you can see that parts of CSE optimization are disabled at -O1:

else if (tem == 1 || optimize > 1)
  cse_cfg_altered |= cleanup_cfg (0);
Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.