Re: [Qemu-devel] [PATCH] tcg: increase MAX_OP_PER_INSTR to 395

[Joseph Myers]

On Fri, 23 Sep 2016, Laurent Desnogues wrote:

> Hello,
> 
> On Fri, Sep 23, 2016 at 1:53 AM, Joseph Myers <address@hidden> wrote:
> > MAX_OP_PER_INSTR is currently 266, reported in commit
> > 14dcdac82f398cbac874c8579b9583fab31c67bf to be the worst case for the
> > ARM A64 decoder.
> >
> > Whether or not it was in fact the worst case at that time in 2014, I'm
> > observing the instruction 0x4c006020 (st1 {v0.16b-v2.16b}, [x1])
> > generate 386 ops from disas_ldst_multiple_struct with current sources,
> 
> Something's odd, I get exactly half of that with 193.

Does the number of ops depend on the system for which TCG is generating 
code?  (I'm building QEMU for 32-bit x86.)  If 32-bit systems require 
twice as many ops as 64-bit systems, maybe the existing value should be 
doubled, so using 532 (plus whatever is needed to allow for extra ops from 
optimization etc.) - or made conditional on the system for which code is 
generated.

> That being said st1 {v0.16b-v3.16b}, [x1], #64 generates even more ops 
> with 258.

My empirical observations are only from examining cases where QEMU gives 
errors running GCC tests; it's quite possible some instructions aren't 
covered, or that the relevant tests got lucky and avoided buffer overruns 
despite generating too many ops.

-- 
Joseph S. Myers
address@hidden