[llvm-dev] (RFC) JumpMaps: switch statement optimization
James Molloy via llvm-dev
llvm-dev at lists.llvm.org
Tue Feb 14 12:55:28 PST 2017
Hi,
Thanks for suggesting this! It sounds like a great optimization.
Hans makes a good point that in some cases inlining the map function could
be very cheap; especially with a very well written linear search loop. I
found this precisely when implementing a similar feature - compressed jump
tables for Thumb-1.
Something to bear in mind when making this flexible in terms of the types
of search functions it can emit is that it's possible, as you alluded
earlier, to overoptimize and end up linking several different search
algorithms and wasting space. This is even a problem with LTO as we do
switch lowering function-at-a-time in isolation.
Sounds Iike a great idea though, I look forward to seeing it in tree!
James
On Tue, 14 Feb 2017 at 18:39, Hans Wennborg via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I wonder if it would make sense to emit the jumpmap_find_ functions in
> IR rather than in compiler-rt.
>
> Many targets don't link against compiler-rt, e.g. x86 Linux typically
> uses libgcc.
>
> If they're emitted in IR, the functions could potentially be inlined.
> For example if the size of the switch is known to be small so no
> binary search is done, could inlining the find_ function be smaller
> than setting up the call?
>
> On Tue, Feb 14, 2017 at 5:47 AM, Witold Waligora via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I didn't answer compiler-rt and benchmarks question.
> >
> > compiler-rt would have to implement the decoding function(s) to undo
> > whatever the compiler did. This would be target-dependent.
> > Our target implements four of them, one for each basic type:
> > jumpmap_find_i8, jumpmap_find_i16, jumpmap_find_i32, jumpmap_find_i64.
> >
> > We don't have any benchmarks for any of the in-tree targets yet.
> >
> > Witold
> >
> >
> > W dniu 2017-02-14 o 14:28, Witold Waligora via llvm-dev pisze:
> >> JumpMap lowering is nearly identical to that of JumpTables with the
> >> exception of lack of range-check basic-block.
> >> We introduce JumpMapInfo structure which follows the same flow as
> >> JumpTableInfo and is finally emitted by AsmPrinter.
> >>
> >> There are many ways a Target may want to encode jumpmaps (deltas,
> >> compression, relative vs absolute), so we plan to keep this flexible and
> >> target-driven when upstreaming.
> >>
> >> Witold
> >>
> >> W dniu 2017-02-14 o 10:53, Nema, Ashutosh pisze:
> >>> Hi Witold,
> >>>
> >>> Can you explain what you meant by adding support in compiler_rt &
> changes in lowering.
> >>>
> >>> Did you tried any benchmarks, please share if you have any numbers
> with performance improvements & code size reduction ?
> >>>
> >>> Regards,
> >>> Ashutosh
> >>>
> >>> -----Original Message-----
> >>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Witold Waligora via llvm-dev
> >>> Sent: Monday, February 13, 2017 6:41 PM
> >>> To: llvm-dev at lists.llvm.org
> >>> Subject: [llvm-dev] (RFC) JumpMaps: switch statement optimization
> >>>
> >>> Hi All,
> >>>
> >>> This is our first appearance on llvm-dev so a small introduction is in
> order.
> >>> We are a team of two working for good few years now on an out-of-tree
> embedded back-end. Our primary goal is code size, secondary performance.
> >>> We've recently implemented an optimization that we believe others
> could use. It's about time to contribute back.
> >>>
> >>> -------------------------------------------------------
> >>> JumpMaps: a generalization of JumpTables JumpTables produce fast and
> small code but have a limitation - they only work if case values are easy
> to compute from the variable being switch()ed. To overcome this limitation
> we introduce a {key,value} structure - a JumpMap.
> >>>
> >>> Simplifying somewhat, LLVM would currently generate a sequential
> >>> if-elseif- for small switches and an inline binary-search for large
> switches, if it can't produce a JumpTable. JumpMaps would instead generate
> a call followed by a jump-through-register.
> >>> Pseudo-code example:
> >>>
> >>> switch(x) {
> >>> case 0x6851: {BB1}
> >>> case 0x1383: {BB2}
> >>> case 0x2224: {BB3}
> >>> default: {defaultBB}
> >>> }
> >>>
> >>> Without Jump Maps:
> >>> if(x==0x6851) {
> >>> goto BB1
> >>> }
> >>> else if(x==0x1383) {
> >>> goto BB2
> >>> }
> >>> else if(x==0x2224){
> >>> goto BB3
> >>> }
> >>> else{
> >>> goto defaultBB
> >>> }
> >>>
> >>> With Jump Maps:
> >>> jumpmap_0 = {
> >>> keys = {3, 0x6851, 0x1383, 0x2224}
> >>> vals = {defaultBB, BB1, BB2, BB3}
> >>> }
> >>> addr dst = __jumpmap_find(&jumpmap_0, x)
> >>> goto dst;
> >>>
> >>> On our target Jump Maps produce both smaller and faster code even for
> quite small switch statements. We believe other architectures would benefit
> as well, X86 and ARM included:
> >>> - jumpmap struct has a good chance of being cached more efficiently
> than a large chunk of inline binary-search code
> >>> - for large switches branch prediction should have an easier time
> compared to inline binary-search
> >>> - embedded architectures (ARM, MSP430) would benefit from smaller code
> (trading off .text for .rodata)
> >>>
> >>> In terms of lowering, JumpMap flow is very similar to that of
> JumpTables. The lowering is even slightly easier as JumpMaps do not require
> an extra range-check basic-block - the default case is also handled by
> __jumpmap_find.
> >>> Targets need to provide lowering for the map structure and
> __jumpmap_find function, which would typically become a part of compier-rt
> library and could be written in hand-crafted asm for extra speed.
> >>>
> >>>
> >>> Questions or comments?
> >>> Please let me know.
> >>>
> >>>
> >>> Best Regards,
> >>> Witold Waligóra
> >>> Myre Laboratories
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> llvm-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>
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> >>
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