Implement -femit-local-var-lifetime which adds local (stack) variable lifetime annotation to LLVM IR, which enables sharing stack space for variables whose lifetimes do not overlap.
Resolves issue #2227
This is not enabled by default yet, to prevent miscompilation due to bugs (should be enabled in future for optimization levels > 0, and when sanitizers are enabled).
And use `IrTypeClass::getVtblType()` to get the vtable LLVM array
type, instead of deriving it from the `IrClass::getVtblSymbol()`
global (invoking that function may define the vtable!).
I've hit some compiler crashes for the Symmetry code base, on
Windows only. I guess the problem surfaced due to
`-link-defaultlib-shared`, which on Windows causes some vtables of
instantiated classes to be defined whenever accessing the vtable
symbol. I don't have a reduced test case unfortunately.
The previous API of returning two `unsigned`s was ambiguous when accessing the second member of a union that is the first member of a struct, where both output were set to zero.
It now returns one `unsigned` and an output `bool` to determine if it should be used as either a field or byte offset.
Make -linkonce-templates *not* tamper with the general template
emission algorithm anymore (so on top of default non-allinst or
-allinst modes), and keep those tweaks as experimental
-linkonce-templates-aggressive.
Compiling the druntime/Phobos unittests is only marginally slowed
down compared to the more aggressive variant (~1.5% for debug,
~2.5% for release). It does show some rough 10% increase in required
memory, but that's in line with non-linkonce-templates.
The more aggressive variant has the advantage of skipping
needsCodegen() and potentially codegen'ing less symbols. The problem
is that if an instantiated symbol isn't explicitly referenced, for
instance a CRT ctor, it might not be codegen'd at all.
The linkage now (2.098) also affects the size of empty structs - e.g.,
1 for extern(C++), 0 for Posix extern(C), 4 for MSVC extern(C).
This also affects beauties like `extern(C) struct S { double[0] a; }`,
as tested by dmd-testsuite's runnable/ldc_cabi1.d, which is hereby
fixed for Posix - don't try to GEP into an empty LL struct.
