Implement ASM_COPY_STACK on ARM #24096
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| asm(" mov sp, %0;\n" | ||
| " mov lr, #0;\n" // Clear link register (lr) and frame pointer | ||
| " mov fp, #0;\n" // (fp) to terminate unwinder. | ||
| " b start_task;\n" // call `start_task` with fake stack frame |
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This assumes that we'll emit .start_task as an asm symbol (despite trying to tell the compiler not to, with use of static and such). What was wrong with using indirect branch?
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Or add lr and fp to the clobbers list
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despite trying to tell the compiler not to
No we don't. That's what the JL_USED_FUNC is for. We are explicitly telling the compiler to emit that symbol.
What was wrong with using indirect branch?
More instructions, but more importantly, I can't find a better way to make sure the compiler don't use, sp, lr or fp as the register to pass in the address. It seems that lr is most likely a valid choise if the compiler want to do that.
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That's what the clobbers list is for, although the compiler may also potentially attempt to save/restore those registers around it. Making the first instruction mov r1, %1 is another way to ensure it, although in that case, using {r1} as the constraint is probably easier.
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using {r1} as the constraint is probably easier.
That's not supported.
That's what the clobbers list is for,
I want to be robust against different compiler options (they might have special behavior on fp). GCC errors when fp is in the clobber list but not for sp and lr.
Making the first instruction
mov r1, %1is another way to ensure it,
Thought about that, which is why I decided that just using a branch directly is simpler. The used attribute is explicitly documented for use like this.
Also change the AArch64 version to use a relative jump instead of loading the address in register. This should make the code robust even if the compiler decide to pick a unusual register (fp or lr) to store the function address. Add `jl_unreachable` so that the compiler can know that the inline asm never returns.
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Also change the AArch64 version to use a relative jump instead of loading the address in register.
This should make the code robust even if the compiler decide to pick a unusual register (fp or lr)
to store the function address.
Add
jl_unreachableso that the compiler can know that the inline asm never returns.