Ah, some other optimization seems to be kicking in here. When I increase the size of the array to > 128 then I see a call to stg_newByteArray# being emitted:
{offset c1kb: // global if ((Sp + -8) < SpLim) (likely: False) goto c1kc; else goto c1kd; c1kc: // global R1 = Main.main1_closure; call (stg_gc_fun)(R1) args: 8, res: 0, upd: 8; c1kd: // global I64[Sp - 8] = c1k9; R1 = 129; Sp = Sp - 8; call stg_newByteArray#(R1) returns to c1k9, args: 8, res: 8, upd: 8; -harendra On Fri, 7 Apr 2023 at 10:49, Harendra Kumar <harendra.ku...@gmail.com> wrote: > Thanks Ben and Carter. > > I compiled the following to Cmm: > > {-# LANGUAGE MagicHash #-} > {-# LANGUAGE UnboxedTuples #-} > > import GHC.IO > import GHC.Exts > > data M = M (MutableByteArray# RealWorld) > > main = do > _ <- IO (\s -> case newByteArray# 1# s of (# s1, arr #) -> (# s1, M > arr #)) > return () > > It produced the following Cmm: > > {offset > c1k3: // global > Hp = Hp + 24; > if (Hp > HpLim) (likely: False) goto c1k7; else goto c1k6; > c1k7: // global > HpAlloc = 24; > R1 = Main.main1_closure; > call (stg_gc_fun)(R1) args: 8, res: 0, upd: 8; > c1k6: // global > I64[Hp - 16] = stg_ARR_WORDS_info; > I64[Hp - 8] = 1; > R1 = GHC.Tuple.()_closure+1; > call (P64[Sp])(R1) args: 8, res: 0, upd: 8; > } > > It seems to be as good as it gets. There is absolutely no scope for > improvement in this. > > -harendra > > On Fri, 7 Apr 2023 at 03:32, Ben Gamari <b...@smart-cactus.org> wrote: > >> Harendra Kumar <harendra.ku...@gmail.com> writes: >> >> > I was looking at the RTS code for allocating small objects via prim ops >> > e.g. newByteArray# . The code looks like: >> > >> > stg_newByteArrayzh ( W_ n ) >> > { >> > MAYBE_GC_N(stg_newByteArrayzh, n); >> > >> > payload_words = ROUNDUP_BYTES_TO_WDS(n); >> > words = BYTES_TO_WDS(SIZEOF_StgArrBytes) + payload_words; >> > ("ptr" p) = ccall allocateMightFail(MyCapability() "ptr", words); >> > >> > We are making a foreign call here (ccall). I am wondering how much >> overhead >> > a ccall adds? I guess it may have to save and restore registers. Would >> it >> > be better to do the fast path case of allocating small objects from the >> > nursery using cmm code like in stg_gc_noregs? >> > >> GHC's operational model is designed in such a way that foreign calls are >> fairly cheap (e.g. we don't need to switch stacks, which can be quite >> costly). Judging by the assembler produced for newByteArray# in one >> random x86-64 tree that I have lying around, it's only a couple of >> data-movement instructions, an %eax clear, and a stack pop: >> >> 36: 48 89 ce mov %rcx,%rsi >> 39: 48 89 c7 mov %rax,%rdi >> 3c: 31 c0 xor %eax,%eax >> 3e: e8 00 00 00 00 call 43 >> <stg_newByteArrayzh+0x43> >> 43: 48 83 c4 08 add $0x8,%rsp >> >> The data movement operations in particular are quite cheap on most >> microarchitectures where GHC would run due to register renaming. I doubt >> that this overhead would be noticable in anything but a synthetic >> benchmark. However, it never hurts to measure. >> >> Cheers, >> >> - Ben >> >
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