[llvm-dev] Improving the split heuristics for the Greedy Register Allocator

[Wei Mi via llvm-dev]

On Wed, Feb 8, 2017 at 6:21 PM, Wei Mi <wmi at google.com> wrote:
> I have an issue that I've been wrestling with for quite some time and I'm
> hoping that someone with a deeper understanding of the register allocator
> can help me with.
>
> Namely, I am trying to teach RA to split a live range rather than
> allocating a CSR. I've attempted a very large number of tweaks to the costs
> (both existing and experimental ones that I've added). However, despite all
> of that, I can't seem to get RA to split the following:
>
>   1 BB#0: derived from LLVM BB %entry
>   2     Live Ins: %X3
>   3         %vreg15<def> = COPY %X3; G8RC:%vreg15
>   4         %vreg4<def> = CMPLDI %vreg15, 0; CRRC:%vreg4 G8RC:%vreg15
>   5         %vreg11:sub_32<def,read-undef> = LI 0; G8RC:%vreg11
>   6         BCC 68, %vreg4, <BB#1>; CRRC:%vreg4
>   7     Successors according to CFG: BB#4(0x30000000 / 0x80000000 = 37.50%)
> BB#1(0x50000000 / 0x80000000 = 62.50%)
>   8
>   9 BB#4:
>  10     Predecessors according to CFG: BB#0
>  11         B <BB#3>
>  12     Successors according to CFG: BB#3(?%)
>  13
>  14 BB#1: derived from LLVM BB %if.end
>  15     Predecessors according to CFG: BB#0
>  16         %vreg6<def> = ADDIStocHA %X2, <ga:@a>; G8RC_and_G8RC_NOX0:%vreg6
>  17         %vreg7<def> = LDtocL <ga:@a>, %vreg6, %X2<imp-use>;
> mem:LD8[GOT] G8RC_and_G8RC_NOX0:%vreg7,%vreg6
>  18         %vreg8<def> = LWA 0, %vreg7;
> mem:LD4[@a](tbaa=!3)(dereferenceable) G8RC:%vreg8 G8RC_and_G8RC_NOX0:%vreg7
>  19         %vreg9<def> = CMPLD %vreg8, %vreg15; CRRC:%vreg9
> G8RC:%vreg8,%vreg15
>  20         BCC 68, %vreg9, <BB#3>; CRRC:%vreg9
>  21         B <BB#2>
>  22     Successors according to CFG: BB#2(0x30000000 / 0x80000000 = 37.50%)
> BB#3(0x50000000 / 0x80000000 = 62.50%)
>  23
>  24 BB#2: derived from LLVM BB %if.then2
>  25     Predecessors according to CFG: BB#1
>  26         ADJCALLSTACKDOWN 96, %R1<imp-def,dead>, %R1<imp-use>
>  27         %vreg16<def> = COPY %vreg15; G8RC:%vreg16,%vreg15
>  28         BL8_NOP <ga:@callVoid>, <regmask **LONG LIST**>,
> %X3<imp-def,dead>
>  29         ADJCALLSTACKUP 96, 0, %R1<imp-def,dead>, %R1<imp-use>
>  30         ADJCALLSTACKDOWN 96, %R1<imp-def,dead>, %R1<imp-use> 31
> %X3<def> = COPY %vreg16; G8RC:%vreg16
>  32         BL8_NOP <ga:@callNonVoid>, <regmask **LONG LIST**>,
> %X3<imp-use>, %X2<imp-use>, %R1<imp-def>, %X3<imp-def>
>  33         ADJCALLSTACKUP 96, 0, %R1<imp-def,dead>, %R1<imp-use>
>  34         %vreg11<def> = COPY %X3; G8RC:%vreg11 35     Successors
> according to CFG: BB#3(?%)
>  36
>  37 BB#3: derived from LLVM BB %return
>  38     Predecessors according to CFG: BB#1 BB#2 BB#4
>  39         %vreg12<def> = EXTSW_32_64 %vreg11:sub_32; G8RC:%vreg12,%vreg11
>  40         %X3<def> = COPY %vreg12; G8RC:%vreg12
>  41         BLR8 %LR8<imp-use>, %RM<imp-use>, %X3<imp-use>
>
> No matter what I do, vreg15 will get a Callee-Saved Register assigned to
> it. However, this is suboptimal. So what I am trying to accomplish is to
> split the live range of vreg15 into the paths without the call and the path
> with the call (BL8_NOP is a call). Then the physical register X3 can be
> used in the paths BB#0 -> BB#1 -> BB#3 and BB#0 -> BB#4 -> BB#3 and it can
> be copied to a Callee-Saved Register in BB#2.
>

Hi Nemanja,

vreg15's live range is not across call. It is weird that it is always
getting CSR. Maybe because of the hint of vreg16, i.e., the
contribution of hint outweigh the cost of CSR first use?

Is it possible to send out the testcase? I can try if there is anyway
to allocate vreg15 to CSR.

Thanks,
Wei.

> Without such a split, vreg15 is assigned a CSR for the entire live range
> and there is no way to avoid having to save/restore the CSR in the
> prologue/epilogue. If one of the two paths that did not actually have the
> call turn out to be the hottest path through the function, there is a lot
> of wasted cycles in the save/restore because we weren't able to shrink-wrap
> this function due to the choice RA made.
>
> If anyone can offer some ideas on what I should do here, I would truly
> appreciate it.
>
> Nemanja