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Hello arm
I use the arm v9-2019-q4-major toolchain. I compile for the STM32H743 (also seen in the setting)
I compile using the following settings. CFLAG contains compiler settings.
CFLAGS += -D__FPU_PRESENTCFLAGS += -DARM_MATH_CM7CFLAGS += -DSTM32CFLAGS += -DSTM32H7CFLAGS += -DSTM32H743xxCFLAGS += -DSTM32H743XIHxCFLAGS += -std=c11CFLAGS += -mthumbCFLAGS += -mcpu=cortex-m7CFLAGS += -mfloat-abi=softfpCFLAGS += -mfpu=fpv5-d16CFLAGS += -gCFLAGS += -fmessage-length=0CFLAGS += -ffunction-sections CFLAGS += -fno-delete-null-pointer-checksCFLAGS += -Wstringop-overflowCFLAGS += -WerrorCFLAGS += -WallCFLAGS += -WextraCFLAGS += -Wno-missing-field-initializersCFLAGS += -Wdouble-promotion
I wonder why I experience different warnings in my code depending on the optimisation?
I have tried with -O0, -O1, -O2, -O3 and -Os.
I find (relevant) warnings - eg warning of variables that might be used non initialised - with -O1, that is not found when compiling using higher levels?
I experience that the number of warnings found increases with the optimisation level. -O3 finds more warnings that -O1.
Best regards Frank.
Actually it is some code (quite a lot to be honest) ported from another target.
I can't post it all here, and I don't have the time to try to make an example with less code.
The warnings I remember to have experienced, is variables that might be used uninitialised (found with -O1 and not with -O2, -O3 and -Os)
" '<my variable>' may be used uninitialized in this function [-Werror=maybe-uninitialized]"
and possible array index out of bound with -O3 only.
"array subscript 237 is above array bounds of 'const DU8 (*[142][3])[20]' {aka 'const unsigned char (*[142][3])[20]'} [-Werror=array-bounds]"
But if hat is expected behaviour, then my question is answered.
I would guess that, at higher optimisation levels, the compiler has to do more in-depth analysis of the source - so that would give it more information on which to give warnings.
Does the documentation mention that default warning levels may be affected by optimisation level ... ?
I would also expect more warnings the higher the optimisation level. That is why I wondered that I see warnings in -O1 that I haven't seen with -O3 or -Os. The manual however pin-point that some optimisations are only used in -O1 and not in higher levels. That could be the reason why.
Yes, the GCC manual mention (at least) one warning that is affected by optimisation level - found searching for "optimization level". It is strict-overflow.
fkl_at_deif said: I see warnings in -O1 that I haven't seen with -O3 or -Os
Do the warnings appear if you specifically enable them?
Do the warnings appear when I specifically enable them in -O3 and -Os?
I haven't tried that: Only -Wall and -Wextra.