* Elaborate on the sub-variants of Variant I.
* Clarify the use of the TCB term.
* Rename a bunch of stuff to be more accurate/descriptive.
* Follow Zig's style around namespacing more.
* Use a structure for the ABI TCB.
No functional change intended.
Accesses to this global variable can require relocations on some platforms (e.g.
MIPS). If we do it before PIE relocations have been applied, we'll crash.
It's actually important for the ABI that r25 (t9) contains the address of the
called function, so that this standard prologue sequence works:
lui $2, %hi(_gp_disp)
addiu $2, $2, %lo(_gp_disp)
addu $gp, $2, $t9
(This is a bit similar to the ToC situation on powerpc that was fixed in
7bc78967b400322a0fc5651f37a1b0428c37fb9d.)
with this rewrite we can call functions inside of
inline assembly, enabling us to use the default start.zig logic
all that's left is to implement lr/sc loops for atomically manipulating
1 and 2 byte values, after which we can use the segfault handler logic.
Switches from using r1 as a temporary to r2. That way, we don't have to set the
`noat` assembler option. (r1 is the scratch register used by the assembler's
pseudoinstructions; the assembler warns when code uses that register explicitly
without `noat` set.)
This prevents it from trying to access thread local storage before it
has set up thread local storage, particularly when code coverage
instrumentation is enabled.
The previous version of this function referenced the argc_argv_ptr global
variable as an inline asm operand. This caused LLVM to generate prologue code to
initialize the ToC so that the global variable can actually be accessed.
Ordinarily, there's nothing wrong with that. But _start() is a naked function!
This makes it actually super surprising that LLVM did this. It also means that
the old version only really worked by accident.
Once the reference to the global variable was removed, no ToC was set up, thus
violating the calling convention once we got to posixCallMainAndExit(). This
then caused any attempt to access global variables here to crash - namely when
setting std.os.linux.elf_aux_maybe.
The fix is to just initialize the ToC manually in _start().
This is problematic for PIE. There's nothing but luck preventing the accesses to
this global variable from requiring relocations. I've observed this being an
issue on MIPS and PowerPC personally, but others may be affected.
Besides, we're really just passing the initial stack pointer value to
posixCallMainAndExit(), so... just do that.
The set of signals that cannot have their action changed is documented in POSIX,
and any additional, non-standard signals are documented by the specific OS. I
see no valid reason why EINVAL should be considered an unpredictable error here.
Andrew Kelley