Some comments:
1. Link wopt.so into cg.so is bad. It breaks the modularization. Also,
the start time is increased at O0/O1 because wopt.so is loaded
unconditionally.
2. It's not good to add this phase to CG since it's target
independent. Maybe you can try to add a new phase to be driver.
3. For case of switch, it's not good to match the patterns to find the
candidates of constant. Maybe you can simplify the high level SCF in
VHO and split this work into 3 parts:
part1: simplify the high level SCF in VHO
part2: simplify the low level BR instructions in the new phase
part3: build the CFG and remove unreachable code.
2012/3/7 Gang Yu <yugang...@gmail.com>:
> Hi, list:
>
> We have a new design and implementation for fix the gcc incompatible
> issue, tracked as bug798,bug588,..., i,e. open64 unable to delete the
> unreachable code under O0 as gcc does.
>
> A typical case below:
>
> int defined_fun()
> {
> }
> int main()
> {
> int t;
> switch(sizeof(t))
> {
> case 1:
> undefined_fun2();
> break;
> case 4:
> defined_fun();
> break;
> case 8:
> undefined_fun3();
> default:
> undefined_fun();
> }
> return 0;
> }
> Previously, we have proposed invoking the wopt by a special preopt phase "q"
> which includes only copy-prop/DCE phases, however, this approach is not
> accepted due to
>
> *) it obviously increase the compile time due to the essetial, compile-time
> expensive components in WOPT. This is not acceptable by O0 users who are
> quite sensitive to the compile-time.
> *) Invoking wopt may cause some stmts losing the dwalf debug info, this is
> not acceptable by people intent to debug the program using "-O0 -g".
>
> After that, we have a detailed analysis on the O0 DCE problem, roughly,
> there are two Unreachable Code Elimination(UCE) cases to handle under O0
> 1) the if -then-else case:
> if (sizeof(type x) == A) {
> ...;
> } else if (sizeof(type y) == B) {
> ...;
> } else {
> ...
> }
> 2). the switch case : just like the above shown case.
>
> under O0, open64 can handle the case 1, this is because (sizeof(type x) ==
> A) can be whirl expressed and folded to
>
> INTCONST 0
> FALSEBR/TRUEBE
>
> so, unreachable branches then can be indirectly deleted by branch handling
> the case V_BR_NONE or V_BR_ALWAYS
>
> All we wanted is to handle the "switch-case", The new suggested way is to
> build CFG at the beginning of CG_Generate_Code, we do *special const
> propagation* and then *condition fold* to make the branch as
>
> U8INTCONST 4 (0x4)
> U8STID 0 <2,2,_temp__switch_index0> T<9,.predef_U8,8> {line: 1/10}
> U8U8LDID 0 <2,2,_temp__switch_index0> T<9,.predef_U8,8>
> U8INTCONST 4 (0x4)
> I4U8EQ
> U8INTCONST 4 be copy propagated to:
> U8U8LDID 0 <2,2,_temp__switch_index0> T<9,.predef_U8,8>
>
> Then
> U8INTCONST 4 (0x4)
> U8INTCONST 4 (0x4)
> I4U8EQ
> folded to
> INTCONST 1
> TRUEBR
>
> then let the branch expansion routine do things.
>
> We have some basic design rules:
> *) make things as simple as possible, the introduced procedure is only for
> gnu compatiblity and no intent to be optimisation. So, we only do limited
> const prop and expression fold, no real DCE or other changes to CFG.
> *) maxium leverage the current production modules, do *not* invent new
> things
> a). invoke the opt_cfg for CFG building
> b). port the OPT_RVI_EMIT for cfg emit and region handling.
>
> Attached is the patch, self checking the patch, we do not find the issues
> by preopt "q" phase, since,
> *) the only compile time expense is to build-up the CFG, then emit back,
> *) we only modify kids of (FALSE/TRUE) branch stmts, do not corrupt the
> dwalf info.
>
> Applying the patch, we build up the linux kernel using "-O0 -g".
>
> Would a gatekeeper please help review the patch? Thanks a lot.
>
>
> Regards
> Gang
>
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>
--
Regards,
Lai Jian-Xin
------------------------------------------------------------------------------
Virtualization & Cloud Management Using Capacity Planning
Cloud computing makes use of virtualization - but cloud computing
also focuses on allowing computing to be delivered as a service.
http://www.accelacomm.com/jaw/sfnl/114/51521223/
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