Re: [EXTERNAL] Unexpected duplicate join points in "Core" output?

In this example: why would it stop being a join point ? Admittedly, my intuition might be skewed by my own ideas about how join points are sortah a semantic special case of other constructs. On Sat, Nov 20, 2021 at 4:17 PM Simon Peyton Jones via ghc-devs < ghc-devs@haskell.org> wrote:

There is absolutely no reason not to common-up those to join points. But we can't common up some join points when we could if they were let's. Consider

join j1 x = x+1 in case v of A -> f (join j2 x = x+1 in ...j2...) B -> ....j1... C -> ....j1...

Even though j2 is identical to j1's, we can't eliminate j2 in favour of j1 because then j1 wouldn't be a join point any more.

GHC.Core.Opt.CSE is conservative at the moment, and never CSE's *any* join point. It would not be hard to make it clever enough to CSE join points, but no one has yet done it.

Do open a ticket!

Simon

PS: I am leaving Microsoft at the end of November 2021, at which point simonpj@microsoft.com will cease to work. Use simon.peytonjones@gmail.com instead. (For now, it just forwards to simonpj@microsoft.com.)

| -----Original Message----- | From: ghc-devs On Behalf Of Viktor | Dukhovni | Sent: 20 November 2021 00:57 | To: ghc-devs@haskell.org | Subject: [EXTERNAL] Unexpected duplicate join points in "Core" output? | | The below "Core" output from "ghc -O2" (9.2/8.10) for the attached | program shows seemingly rendundant join points: | | join { | exit :: State# RealWorld -> (# State# RealWorld, () #) | exit (ipv :: State# RealWorld) = jump $s$j ipv } in | | join { | exit1 :: State# RealWorld -> (# State# RealWorld, () #) | exit1 (ipv :: State# RealWorld) = jump $s$j ipv } in | | that are identical in all but name. These correspond to fallthrough to | the "otherwise" case in: | | ... | | acc < q || (acc == q && d <= 5) | -> loop (ptr `plusPtr` 1) (acc * 10 + d) | | otherwise -> return Nothing | | but it seems that the generated X86_64 code (also below) ultimately | consolidates these into a single target... Is that why it is harmless | to leave these duplicated in the generated "Core"? | | [ Separately, in the generated machine code, it'd also be nice to avoid | comparing the same "q" with the accumulator twice. A single load and | compare should I think be enough, as I'd expect the status flags to | persist across the jump the second test. | | This happens to not be performance critical in my case, because most | calls should satisfy the first test, but generally I think that 3-way | "a < b", "a == b", "a > b" branches ideally avoid comparing twice... | ] | | ======== Associated Core output | | -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0} | main2 :: Addr# | main2 = "12345678901234567890 junk"# | | -- RHS size: {terms: 129, types: 114, coercions: 0, joins: 6/8} | main1 :: State# RealWorld -> (# State# RealWorld, () #) | main1 | = \ (eta :: State# RealWorld) -> | let { | end :: Addr# | end = plusAddr# main2 25# } in | join { | $s$j :: State# RealWorld -> (# State# RealWorld, () #) | $s$j _ = hPutStr2 stdout $fShowMaybe4 True eta } in | join { | exit :: State# RealWorld -> (# State# RealWorld, () #) | exit (ipv :: State# RealWorld) = jump $s$j ipv } in | join { | exit1 :: State# RealWorld -> (# State# RealWorld, () #) | exit1 (ipv :: State# RealWorld) = jump $s$j ipv } in | join { | exit2 | :: Addr# -> Word# -> State# RealWorld -> (# State# | RealWorld, () #) | exit2 (ww :: Addr#) (ww1 :: Word#) (ipv :: State# | RealWorld) | = case eqAddr# ww main2 of { | __DEFAULT -> | hPutStr2 | stdout | (++ | $fShowMaybe1 | (case $w$cshowsPrec3 11# (integerFromWord# | ww1) [] of | { (# ww3, ww4 #) -> | : ww3 ww4 | })) | True | eta; | 1# -> jump $s$j ipv | } } in | joinrec { | $wloop | :: Addr# -> Word# -> State# RealWorld -> (# State# | RealWorld, () #) | $wloop (ww :: Addr#) (ww1 :: Word#) (w :: State# RealWorld) | = join { | getDigit :: State# RealWorld -> (# State# RealWorld, | () #) | getDigit (eta1 :: State# RealWorld) | = case eqAddr# ww end of { | __DEFAULT -> | case readWord8OffAddr# ww 0# eta1 of { (# | ipv, ipv1 #) -> | let { | ipv2 :: Word# | ipv2 = minusWord# (word8ToWord# ipv1) 48## | } in | case gtWord# ipv2 9## of { | __DEFAULT -> | case ltWord# ww1 1844674407370955161## of | { | __DEFAULT -> | case ww1 of { | __DEFAULT -> jump exit ipv; | 1844674407370955161## -> | case leWord# ipv2 5## of { | __DEFAULT -> jump exit1 ipv; | 1# -> | jump $wloop | (plusAddr# ww 1#) | (plusWord# | 18446744073709551610## ipv2) | ipv | } | }; | 1# -> | jump $wloop | (plusAddr# ww 1#) (plusWord# | (timesWord# ww1 10##) ipv2) ipv | }; | 1# -> jump exit2 ww ww1 ipv | } | }; | 1# -> jump exit2 ww ww1 eta1 | } } in | jump getDigit w; } in | jump $wloop main2 0## realWorld# | | ======== Executable disassembly | | The jumps at "-1->" and "-2->" that correspond that "otherwise" have | the same target. The duplicate "load+cmp" with "q" is at "-3->" and "- | 4->": | | 0000000000408de8 : | 408de8: 48 8d 45 e8 lea -0x18(%rbp),%rax | 408dec: 4c 39 f8 cmp %r15,%rax | 408def: 0f 82 c8 00 00 00 jb 408ebd | | 408df5: b8 79 dd 77 00 mov $0x77dd79,%eax | 408dfa: 31 db xor %ebx,%ebx | 408dfc: b9 60 dd 77 00 mov $0x77dd60,%ecx | 408e01: 48 39 c1 cmp %rax,%rcx | 408e04: 74 66 je 408e6c | | 408e06: 0f b6 11 movzbl (%rcx),%edx | 408e09: 48 83 c2 d0 add | $0xffffffffffffffd0,%rdx | 408e0d: 48 83 fa 09 cmp $0x9,%rdx | 408e11: 77 59 ja 408e6c | | -3-> 408e13: 48 be 99 99 99 99 99 mov | $0x1999999999999999,%rsi | 408e1a: 99 99 19 | 408e1d: 48 39 f3 cmp %rsi,%rbx | 408e20: 73 0c jae 408e2e | | 408e22: 48 6b db 0a imul $0xa,%rbx,%rbx | 408e26: 48 01 d3 add %rdx,%rbx | 408e29: 48 ff c1 inc %rcx | 408e2c: eb d3 jmp 408e01 | | -4-> 408e2e: 48 be 99 99 99 99 99 mov | $0x1999999999999999,%rsi | 408e35: 99 99 19 | 408e38: 48 39 f3 cmp %rsi,%rbx | -1-> 408e3b: 75 49 jne 408e86 | | 408e3d: 48 83 fa 05 cmp $0x5,%rdx | -2-> 408e41: 77 43 ja 408e86 | | 408e43: 48 8d 5a fa lea -0x6(%rdx),%rbx | 408e47: 48 ff c1 inc %rcx | 408e4a: eb b5 jmp 408e01 | | 408e4c: 0f 1f 40 00 nopl 0x0(%rax) | 408e50: c2 00 00 retq $0x0 | | -- | Viktor. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs