memtest86+ EFI » GDB debug script for memtest86+

Explanation in gcc-documentation:

-Os Optimize for size. -Os enables all -O2 optimizations except those that often increase code size

-O3 Highest opmtimization level

Good choice:
-Og Optimize debugging experience. Enables all -O1 optimization flags except for those that may interfere with debugging

Without changes (original build procedure), memtest.efi has 141K

Changing the optimization level results in resolving errors:

system/smp.o: In function `smp_init':
/home/koenigr/Memtest/git/memtest86plus_debug/build64/../system/smp.c:558: undefined reference to `memcpy'
system/xhci.o: In function `xhci_init':
/home/koenigr/Memtest/git/memtest86plus_debug/build64/../system/xhci.c:1017: undefined reference to `memset'
Makefile:133: recipe for target 'memtest_shared' failed
make: *** [memtest_shared] Error 1

lib/String.h:

memcpy is __builtin_memcpy
memset is __builtin_memset

These are the only two functions defined this way

nm lib/string.o

But they are also not defined when compiling with official optimization flags

https://stackoverflow.com/questions/27850184/g-gcc-compiler-is-giving-undefined-reference-error-when-optiomzation-is-turned
gives a hint:

quite simple: if you enable optimization the compiler inlines the function. There is no need to link against the library
function. If optimization is switched of, you need the function from the lib. Solution: Link with your library which
contains the needed function! You have to add something like

g++    -g -O3 use_library.cpp -L<LIBDIR> -l<libname>

https://stackoverflow.com/questions/11747891/when-builtin-memcpy-is-replaced-with-libcs-memcpy
gives second hint:

When I should prefer of using __builtin_memcpy over plain memcpy?

It appears that it uses an inline version when the size is a compile time constant and it's less than or equal to 8192.