Comment 14 for bug 1738730

I've found the holy grail. Problem is happening if you are using a pointer to a struct. Normally, while accessing to that struct members, nothing is wrong, it accesses word by word. But if you declare a pointer to that struct and do your business with it, problem arises and GCC starts using byte by byte method even while unaligned access is enabled.

Here's worse part: It doesn't matter whether you use -fpack-struct flag or #pragma pack(push, 1). You'll get the same result.

Please find the simplest example as attachment. And use this line (don't forget to add your toolchains path in the beginning)

arm-none-eabi-g++ -gdwarf-2 -mcpu=cortex-m3 -mthumb -std=c++1y '-DDEBUG=1' -O0 -pedantic -Wall -Wextra -c -fmessage-length=0 -fno-rtti -fno-exceptions -mno-sched-prolog -fno-builtin -fshort-enums -ffunction-sections -fdata-sections -MMD -MP -MF"main.d" -MT"main.o" -S -o "main.s" "main.cpp"

Here's the output of the important part:

.LCFI2:
 .cfi_def_cfa_register 7
 .loc 1 17 0
 ldr r2, .L3
 mov r3, r7
 str r3, [r2]
 .loc 1 19 0
 ldr r3, .L3
 ldr r3, [r3]
 ldrb r2, [r3]
 movs r2, #0
 orr r2, r2, #1
 strb r2, [r3]
 ldrb r2, [r3, #1]
 movs r2, #0
 orr r2, r2, #4
 strb r2, [r3, #1]
 ldrb r2, [r3, #2]
 movs r2, #0
 strb r2, [r3, #2]
 ldrb r2, [r3, #3]
 movs r2, #0
 strb r2, [r3, #3]

I would be glad if anyone can pinpoint the root cause. Things should be gone bad between 4.9 and 5.4.

Regards,