Be less conservative wrt. linker dead code elimination

[kinke]

No description provided.

[kinke]

Ouch, Apple's ld64 apparently strips rt_options globals from executables linked against shared druntime. This might be a blocker for Apple targets.

[kinke]

Very unsure about the last [tentative fix] commit - has this really just worked accidentally so far (and only started to show up on macOS with shared druntime now with dead_strip)? It's about

ldc/runtime/druntime/test/profile/src/profile.d

Lines 17 to 19 in 95ac5ca

	// Issue 19593
	assert(!("aaa" >= "bbb"));
	assert("aaa" <= "bbb");

[kinke]

The size of the bundled D executables (except for ldc2 and ldc-profdata, which are unchanged) from the macOS-x64 CI package is cut in half - from 59 MB to 30.4 MB; e.g., ldmd2 from 4.4 to 1.2 MB.

[JohanEngelen]

Very unsure about the last [tentative fix] commit - has this really just worked accidentally so far (and only started to show up on macOS with shared druntime now with dead_strip)? It's about

ldc/runtime/druntime/test/profile/src/profile.d

Lines 17 to 19 in 95ac5ca

	// Issue 19593
	assert(!("aaa" >= "bbb"));
	assert("aaa" <= "bbb");

Sorry I don't fully understand. Why was the fix needed? (what does adding check=assert=on change for this test?)

[kinke]

The produced .def file didn't include the two string-compare functions (dstrcmp and __cmp) expected in https://github.com/ldc-developers/ldc/blob/95ac5cadb89e2fde1d9501f3aac2dc8a077c5278/runtime/druntime/test/profile/mytrace.def.exp - on macOS only, and only when linking shared druntime (tested by 2 CI jobs, both failed), and only with dead_strip. I don't know anything about how this profiling code works; and how the 2 functions were apparently in there before the dead_strip, even though the string comparisons are AFAICT exclusively triggered behind assertions, which are ignored with -release when running that test in release mode.

[JohanEngelen]

The produced .def file didn't include the two string-compare functions (dstrcmp and __cmp) expected in https://github.com/ldc-developers/ldc/blob/95ac5cadb89e2fde1d9501f3aac2dc8a077c5278/runtime/druntime/test/profile/mytrace.def.exp - on macOS only, and only when linking shared druntime (tested by 2 CI jobs, both failed), and only with dead_strip. I don't know anything about how this profiling code works; and how the 2 functions were apparently in there before the dead_strip, even though the string comparisons are AFAICT exclusively triggered behind assertions, which are ignored with -release when running that test in release mode.

yeah ok... I'm OK with your fix :)