On 2 February 2011 10:47, Dave Martin <dave.mar...@linaro.org> wrote: > On Tue, Feb 1, 2011 at 12:33 PM, David Gilbert <david.gilb...@linaro.org> > wrote: >> Hi, >> What do people understand to be the expected semantics of IT blocks >> in the cases below, of which there has been some confusion >> in relation to a recent Qt issue. >> >> The code in question had a sequence something like: >> >> >> comparison >> IT... EQ >> blahEQ >> TEQ >> BEQ >> >> The important bits here are that we have an IT EQ block and two special >> cases: >> >> 1) There is a TEQ in the IT block - are all comparisons in the block >> allowed and do their effects immediately take >> effect? As far as I can tell this is allowed and any flag changes are >> used straight away; > > Yes; yes; and: you're right. This was a specific intention, since > there was always a common idiom on ARM of sequences like this: > > CMP r0, #1 > CMPEQ r1, #2 > CMPEQ r2, #3 > BEQ ... > > with the effect of "if(r0==1 && r1==2 && r2==3) ..." > >> >> 2) There is a BEQ at the end of the IT block, as far as I can tell, >> as long as the destination of the BEQ is close it shouldn't >> make any difference if the BEQ is included in the IT block or not. > > Again, I believe you're right. The assembler will generate different > code, because the explicit conditional branch encodings are not > allowed in IT blocks. But the assembler takes care of this for you: > > 00000000 <f>: > 0: d001 beq.n 6 <g> > > versus > > 2: bf08 it eq > 4: e7ff beq.n 6 <g> > > 00000006 <g>: > > Both snippets are equivalent, though as you say, with IT you can > insert more code between the branch and its destination before the > assembler will barf with a fixup overflow, because the unconditional > branch encoding (e000..e7fff) has more bits to express the branch > offset.
Thanks for the confirmation Dave. Dave _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
