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. Cheers ---Dave _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
