Detection of warning depend on optimisation

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_PRESENT
CFLAGS  += -DARM_MATH_CM7
CFLAGS  += -DSTM32
CFLAGS  += -DSTM32H7
CFLAGS  += -DSTM32H743xx
CFLAGS  += -DSTM32H743XIHx
CFLAGS  += -std=c11
CFLAGS  += -mthumb
CFLAGS  += -mcpu=cortex-m7
CFLAGS  += -mfloat-abi=softfp
CFLAGS  += -mfpu=fpv5-d16
CFLAGS  += -g
CFLAGS  += -fmessage-length=0
CFLAGS  += -ffunction-sections
CFLAGS  += -fno-delete-null-pointer-checks

CFLAGS  += -Wstringop-overflow

CFLAGS  += -Werror
CFLAGS  += -Wall
CFLAGS  += -Wextra

CFLAGS  += -Wno-missing-field-initializers
CFLAGS  += -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.

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  • I think you'd need to show some actual code and the actual warnings you get to get meaningful answers - else we're just guessing.

    I experience that the number of warnings found increases with the optimisation level. -O3 finds more warnings that -O1.

    I don't think that would be a surprise?

    It is common that you can "get away" with some questionable coding at lower optimisation levels, but they will "bite" you at higher levels.

    An obvious example would be the use of 'volatile' ...

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  • I think you'd need to show some actual code and the actual warnings you get to get meaningful answers - else we're just guessing.

    I experience that the number of warnings found increases with the optimisation level. -O3 finds more warnings that -O1.

    I don't think that would be a surprise?

    It is common that you can "get away" with some questionable coding at lower optimisation levels, but they will "bite" you at higher levels.

    An obvious example would be the use of 'volatile' ...

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