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straight-shoota merged 1 commit into from
Mar 24, 2025
Merged

ARM: reduce duplication in fiber context switch #15585

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135 changes: 54 additions & 81 deletions src/fiber/context/arm.cr
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Original file line number Diff line number Diff line change
Expand Up @@ -21,100 +21,73 @@ class Fiber
@[NoInline]
@[Naked]
def self.swapcontext(current_context, new_context) : Nil
# r0 , r1

# ARM assembly requires integer literals to be moved to a register before
# being stored at an address; we use r4 as a scratch register that will be
# overwritten by the new context.
#
# Eventually reset LR to zero to avoid the ARM unwinder to mistake the
# context switch as a regular call.

{% if flag?(:armhf) %}
# r0 , r1
{% if compare_versions(Crystal::LLVM_VERSION, "9.0.0") >= 0 %}
asm("
// declare the presence of a conservative FPU to the ASM compiler
.fpu vfp

stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
vstmdb sp!, {d8-d15} // push FPU registers
str sp, [r0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [r0, #4]

mov r4, #0 // new_context.resumable = 0
str r4, [r1, #4]
ldr sp, [r1, #0] // sp = new_context.stack_top
vldmia sp!, {d8-d15} // pop FPU registers
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers
{% if compare_versions(Crystal::LLVM_VERSION, "9.0.0") >= 0 %}
asm("
{% if flag?(:armhf) %}
// declare the presence of a conservative FPU to the ASM compiler
.fpu vfp
{% end %}

// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
")
{% else %}
# On LLVM < 9.0 using the previous code emits some additional
# instructions that breaks the context switching.
asm("
// declare the presence of a conservative FPU to the ASM compiler
.fpu vfp
stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
{% if flag?(:armhf) %}
vstmdb sp!, {d8-d15} // push FPU registers
{% end %}
str sp, [r0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [r0, #4]

stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
vstmdb sp!, {d8-d15} // push FPU registers
str sp, [$0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [$0, #4]
mov r4, #0 // new_context.resumable = 0
str r4, [r1, #4]
ldr sp, [r1, #0] // sp = new_context.stack_top
{% if flag?(:armhf) %}
vldmia sp!, {d8-d15} // pop FPU registers
{% end %}
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers

mov r4, #0 // new_context.resumable = 0
str r4, [$1, #4]
ldr sp, [$1, #0] // sp = new_context.stack_top
vldmia sp!, {d8-d15} // pop FPU registers
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers

// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
" :: "r"(current_context), "r"(new_context))
{% end %}
{% elsif flag?(:arm) %}
{% if compare_versions(Crystal::LLVM_VERSION, "9.0.0") >= 0 %}
asm("
stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
str sp, [r0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [r0, #4]

mov r4, #0 // new_context.resumable = 0
str r4, [r1, #4]
ldr sp, [r1, #0] // sp = new_context.stack_top
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers
// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
")
{% else %}
# On LLVM < 9.0 using the previous code emits some additional
# instructions that breaks the context switching.
asm("
{% if flag?(:armhf) %}
// declare the presence of a conservative FPU to the ASM compiler
.fpu vfp
{% end %}

// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
")
{% else %}
# On LLVM < 9.0 using the previous code emits some additional
# instructions that breaks the context switching.
asm("
stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
str sp, [$0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [$0, #4]
stmdb sp!, {r0, r4-r11, lr} // push 1st argument + callee-saved registers
{% if flag?(:armhf) %}
vstmdb sp!, {d8-d15} // push FPU registers
{% end %}
str sp, [$0, #0] // current_context.stack_top = sp
mov r4, #1 // current_context.resumable = 1
str r4, [$0, #4]

mov r4, #0 // new_context.resumable = 0
str r4, [$1, #4]
ldr sp, [$1, #0] // sp = new_context.stack_top
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers
mov r4, #0 // new_context.resumable = 0
str r4, [$1, #4]
ldr sp, [$1, #0] // sp = new_context.stack_top
{% if flag?(:armhf) %}
vldmia sp!, {d8-d15} // pop FPU registers
{% end %}
ldmia sp!, {r0, r4-r11, lr} // pop 1st argument + callee-saved registers

// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
" :: "r"(current_context), "r"(new_context))
{% end %}
// avoid a stack corruption that will confuse the unwinder
mov r1, lr
mov lr, #0
mov pc, r1
" :: "r"(current_context), "r"(new_context))
{% end %}
end
end
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