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start: A handful of MIPS fixes #20822

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merged 4 commits into from
Jul 29, 2024
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start: A handful of MIPS fixes #20822

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77fc3b8
start: Set up the gp register on mips and mips64.
alexrp Jul 24, 2024
e33af8e
start: Perform the posixCallMainAndExit() call with jalr on mips.
alexrp Jul 25, 2024
d6c637c
start: Set std.os.linux.elf_aux_maybe after PIE relocations.
alexrp Jul 24, 2024
d633b35
start: Always inline the call to std.os.linux.pie.relocate().
alexrp Jul 25, 2024
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41 changes: 35 additions & 6 deletions lib/std/start.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -330,17 +330,42 @@ fn _start() callconv(.Naked) noreturn {
,
.mips, .mipsel =>
\\ move $fp, $0
\\ bal 1f
\\ .gpword .
\\ .gpword %[posixCallMainAndExit]
\\ 1:
\\ lw $gp, 0($ra)
\\ subu $gp, $ra, $gp
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\\ lw $25, 4($ra)
\\ addu $25, $25, $gp
\\ move $ra, $0
\\ move $a0, $sp
\\ and $sp, -8
\\ j %[posixCallMainAndExit]
\\ subu $sp, $sp, 16
\\ jalr $25
,
.mips64, .mips64el =>
\\ move $fp, $0
// This is needed because early MIPS versions don't support misaligned loads. Without
// this directive, the hidden `nop` inserted to fill the delay slot after `bal` would
// cause the two doublewords to be aligned to 4 bytes instead of 8.
\\ .balign 8
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\\ bal 1f
\\ .gpdword .
\\ .gpdword %[posixCallMainAndExit]
\\ 1:
// The `gp` register on MIPS serves a similar purpose to `r2` (ToC pointer) on PPC64.
// We need to set it up in order for dynamically-linked / position-independent code to
// work.
\\ ld $gp, 0($ra)
\\ dsubu $gp, $ra, $gp
\\ ld $25, 8($ra)
\\ daddu $25, $25, $gp
\\ move $ra, $0
\\ move $a0, $sp
\\ and $sp, -16
\\ j %[posixCallMainAndExit]
\\ dsubu $sp, $sp, 16
\\ jalr $25
,
.powerpc, .powerpcle =>
// Set up the initial stack frame, and clear the back chain pointer.
Expand Down Expand Up @@ -430,7 +455,6 @@ fn posixCallMainAndExit(argc_argv_ptr: [*]usize) callconv(.C) noreturn {
if (native_os == .linux) {
// Find the beginning of the auxiliary vector
const auxv: [*]elf.Auxv = @ptrCast(@alignCast(envp.ptr + envp_count + 1));
std.os.linux.elf_aux_maybe = auxv;

var at_hwcap: usize = 0;
const phdrs = init: {
Expand All @@ -448,12 +472,17 @@ fn posixCallMainAndExit(argc_argv_ptr: [*]usize) callconv(.C) noreturn {
break :init @as([*]elf.Phdr, @ptrFromInt(at_phdr))[0..at_phnum];
};

// Apply the initial relocations as early as possible in the startup
// process.
// Apply the initial relocations as early as possible in the startup process. We cannot
// make calls yet on some architectures (e.g. MIPS) *because* they haven't been applied yet,
// so this must be fully inlined.
if (builtin.position_independent_executable) {
std.os.linux.pie.relocate(phdrs);
@call(.always_inline, std.os.linux.pie.relocate, .{phdrs});
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}

// This must be done after PIE relocations have been applied or we may crash
// while trying to access the global variable (happens on MIPS at least).
std.os.linux.elf_aux_maybe = auxv;

if (!builtin.single_threaded) {
// ARMv6 targets (and earlier) have no support for TLS in hardware.
// FIXME: Elide the check for targets >= ARMv7 when the target feature API
Expand Down