32-bit Thumb-2 breakpoints
dan at debian.org
Mon Jan 11 18:17:02 EST 2010
On Mon, Jan 11, 2010 at 11:10:27PM +0000, Jamie Lokier wrote:
> Isn't the itt sequence for Thumb-2 supposed to be consistent with the
> condition sequence for ARM, so that it should be:
> cmp r0, r0
> itt eq
> A: cmpeq r0, r1
> B: movne r0, r2 <- that is, ne not eq, because it's the "then" branch
> C: nop
No, that'd be if you wrote "ite eq". This is a more complicated
sequence; you can get the ARM equivalent by ignoring the itt
instruction (the assembler will just use it for a consistency check).
This block is "(r0 == r0 && r0 == r1) -> r0 = r2".
> And does the cmp at A really affect execution of the instructions at B
> and C in a Thumb-2 itt sequence, rather than the condition being
> checked once at the time of the itt instruction and played out for the
> following 3 instructions?
Yes, it really does. Useful for compound conditions, and for maximal
confusion. Thankfully, both branches into IT blocks and branches
leaving IT blocks not at the last instruction are forbidden.
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