-Omax and LTO have no mapping/abstraction yet

[ReinhardKeil]

Describe The Problem To Be Solved

AC6 has -Omax and several compilers offer Link Time Optimization (LTO). We should evaluate if we can add features for it.

See also ARM-software/vscode-cmsis-csolution#131

[brondani]

What generic optimize value should be added?

Can we precisely establish what should be implemented for each supported toolchain?
For example:

Toolchain	Compiler and/or Linker options	Doc
AC6	-Omax	1
GCC	-O3 -flto	2
CLANG	-O3 -flto	3
CLANG_TI	-O3 -flto	4
IAR	-Oh? --vfe --inline --merge_duplicate_sections	5

Please note Arm states that -Omax "might violate strict compliance with language standards":
https://developer.arm.com/documentation/100748/0623/Using-Common-Compiler-Options/Selecting-optimization-options

[ReinhardKeil]

Maybe the best is to add another control to enable link-time-optimize:.

[mario-pi]

IAR compiler optimizations can be directed for size (-Ohz), speed (-Ohs) or balanced (-Oh). The linker has no general -O option, but it can enable specific optimizations. I discussed this with the compiler team:

--vfe is a size optimization (discards unused virtual functions), the cost is link time (minimal) so it should be used

--inline inlines small functions (the payload of the target fits in the code). This is a speed optimization. This can be problematic if you place a breakpoint in foobar, and then it is optimized away so that the call to foobar never takes place. In EW this is default on for Release build configurations.

--merge_duplicate_sections is a size optimization, like the name implies duplicate sections are eliminated. This is a relatively expensive (paid with link time) optimization. It is not active by default in EW.

Generally speaking I agree with @ReinhardKeil that it would probably be best to have a separate control for linker optimization, mostly given that they are so specific.

[ReinhardKeil]

Proposal separate option:

Also AC6 offers the option --flto. As this can be combined with optimize levels (size, speed, etc.) it should be a separate option, my proposal link-time-optimize:. For the files or components it is applied it enables --flto. When one or more files use this option a corresponding linker option should be enabled.

IAR compiler uses multi-file compilation which at this point in time cannot handled by CMake. Therefore this option should be ignored for IAR.