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Fix greenlet on aarch64 with gcc 4.8 #29

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Apr 22, 2013
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Fix greenlet on aarch64 with gcc 4.8 #29

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39 changes: 35 additions & 4 deletions platform/switch_aarch64_gcc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
* this is the internal transfer function.
*
* HISTORY
* 13-Apr-13 Add support for strange GCC caller-save decisions
* 08-Apr-13 File creation. Michael Matz
*
* NOTES
Expand All @@ -19,13 +20,22 @@
"v8", "v9", "v10", "v11", \
"v12", "v13", "v14", "v15"

/* See below for the purpose of this function. */
__attribute__((noinline, noclone)) int fancy_return_zero(void);
__attribute__((noinline, noclone)) int
fancy_return_zero(void)
{
return 0;
}

static int
slp_switch(void)
{
int err = 0;
void *fp;
register int *stackref, stsizediff;
register long *stackref, stsizediff;
__asm__ volatile ("" : : : REGS_TO_SAVE);
__asm__ volatile ("str x29, %0" : "=m"(fp) : :);
__asm__ volatile ("str x29, %0" : "=m"(fp) : : );
__asm__ ("mov %0, sp" : "=r" (stackref));
{
SLP_SAVE_STATE(stackref, stsizediff);
Expand All @@ -35,11 +45,32 @@ slp_switch(void)
:
: "r" (stsizediff)
);
SLP_RESTORE_STATE();
SLP_RESTORE_STATE();
/* SLP_SAVE_STATE macro contains some return statements
(of -1 and 1). It falls through only when
the return value of slp_save_state() is zero, which
is placed in x0.
In that case we (slp_switch) also want to return zero
(also in x0 of course).
Now, some GCC versions (seen with 4.8) think it's a
good idea to save/restore x0 around the call to
slp_restore_state(), instead of simply zeroing it
at the return below. But slp_restore_state
writes random values to the stack slot used for this
save/restore (from when it once was saved above in
SLP_SAVE_STATE, when it was still uninitialized), so
"restoring" that precious zero actually makes us
return random values. There are some ways to make
GCC not use that zero value in the normal return path
(e.g. making err volatile, but that costs a little
stack space), and the simplest is to call a function
that returns an unknown value (which happens to be zero),
so the saved/restored value is unused. */
err = fancy_return_zero();
}
__asm__ volatile ("ldr x29, %0" : : "m" (fp) :);
__asm__ volatile ("" : : : REGS_TO_SAVE);
return 0;
return err;
}

#endif