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Re: thumb-2 and crt0.S init_hooks

Richard Earnshaw wrote:
On Thu, 2008-12-18 at 16:06 -0200, Alexandre Pereira Nunes wrote:

It seems that there's a problem with the patch suggested on

This sort of constructs:

+#if defined(__thumb__) || defined(__thumb2__)
+	blx   r3
 	mov	lr, pc
 	mov	pc, r3

Will probably fail on armv4t targets. The reason is that this
architecture lacks support for the blx instruction.
Also, on thumb1 targets, mov lr, pc will do right thing, because the
calling code is in arm mode, but the mov pc, r3 may fail to call a
thumb function.


The way I see it, this sequence will probably work fine:

+#if defined(__thumb2__) + blx r3 +#else mov lr, pc bx r3 +#endif

But there's still a catch on armv4t targets, if the code is compiled
with -mno-thumb-interwork (default for arm-elf targets, but not for
arm-*-eabi ones): some gcc versions tends to emit mov pc, lr or
similar sequences in order to return from thumb functions, and on this
case, a thunk has to be implemented to manually switch from thumb to
arm mode since the processor is unable to do this by itself.

Nope, not right either...

1) Pre-armv4t doesn't have BX.
2) in Thumb state, mov lr, pc doesn't work as expected, because the T
bit is stripped out of the PC value.
3) On armv5t or later, there is a blx instruction in both ARM and Thumb
code; when this exists, it's always preferable to the the
two-instruction alternative.


Richard, would you like to propose a patch? I'm just about to build newlib 1.17.0 but I can wait
if there is a patch for this problem.

-- Jeff J.

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