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start: Avoid needlessly aligning the stack pointer on some architectures. #20737

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start: Avoid needlessly aligning the stack pointer on some architectures. #20737

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21 changes: 1 addition & 20 deletions lib/std/start.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -276,13 +276,11 @@ fn _start() callconv(.Naked) noreturn {
.x86_64 =>
\\ xorl %%ebp, %%ebp
\\ movq %%rsp, %%rdi
\\ andq $-16, %%rsp
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@alexrp alexrp Jul 22, 2024

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\\ callq %[posixCallMainAndExit:P]
,
.x86 =>
\\ xorl %%ebp, %%ebp
\\ movl %%esp, %%eax
\\ andl $-16, %%esp
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@alexrp alexrp Jul 22, 2024

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\\ subl $12, %%esp
\\ pushl %%eax
\\ calll %[posixCallMainAndExit:P]
Expand All @@ -307,26 +305,10 @@ fn _start() callconv(.Naked) noreturn {
\\ andi sp, sp, -16
\\ tail %[posixCallMainAndExit]@plt
,
.mips, .mipsel =>
.mips, .mipsel, .mips64, .mips64el =>
// The lr is already zeroed on entry, as specified by the ABI.
\\ addiu $fp, $zero, 0
\\ move $a0, $sp
\\ .set push
\\ .set noat
\\ addiu $1, $zero, -16
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@alexrp alexrp Jul 22, 2024

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No authoritative document seems to exist for 64-bit MIPS, but: https://github.com/torvalds/linux/blob/933069701c1b507825b514317d4edd5d3fd9d417/arch/mips/kernel/process.c#L708-L718

\\ and $sp, $sp, $1
\\ .set pop
\\ j %[posixCallMainAndExit]
,
.mips64, .mips64el =>
// The lr is already zeroed on entry, as specified by the ABI.
\\ addiu $fp, $zero, 0
\\ move $a0, $sp
\\ .set push
\\ .set noat
\\ daddiu $1, $zero, -16
\\ and $sp, $sp, $1
\\ .set pop
\\ j %[posixCallMainAndExit]
,
.powerpc, .powerpcle =>
Expand All @@ -343,7 +325,6 @@ fn _start() callconv(.Naked) noreturn {
\\ addis 2, 12, .TOC. - _start@ha
\\ addi 2, 2, .TOC. - _start@l
\\ mr 3, 1
\\ clrrdi 1, 1, 4
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@alexrp alexrp Jul 22, 2024

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\\ li 0, 0
\\ stdu 0, -32(1)
\\ mtlr 0
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