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<p>Hello Victor,</p>
<p>generally GHC does try to common up join points and duplicate
expressions like that.<br>
But since that's relatively expensive most of the duplication
happens during the core-cse pass which only happens once.<br>
<br>
We don't create them because they are harmless. They are simple a
side product of optimizations happening after<br>
the main cse pass has run. There is no feasible way to fix this I
think. As you say with some luck they get caught at the Cmm stage
and<br>
deduplicated there. Sadly it doesn't always happen. In most cases
the impact of this is thankfully rather<br>
small.</p>
<p>For the assembly I opened a ticket:
<a class="moz-txt-link-freetext" href="https://gitlab.haskell.org/ghc/ghc/-/issues/20714">https://gitlab.haskell.org/ghc/ghc/-/issues/20714</a><br>
</p>
<div class="moz-cite-prefix">Am 20/11/2021 um 02:02 schrieb Viktor
Dukhovni:<br>
</div>
<blockquote type="cite"
cite="mid:YZhJOT1T%2FexBDqse@straasha.imrryr.org">
<pre class="moz-quote-pre" wrap="">[ Sorry wrong version of attachment in previous message. ]
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 <Main_main1_info>:
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 <Main_main1_info+0xd5>
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 <Main_main1_info+0x84>
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 <Main_main1_info+0x84>
-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 <Main_main1_info+0x46>
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 <Main_main1_info+0x19>
-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 <Main_main1_info+0x9e>
408e3d: 48 83 fa 05 cmp $0x5,%rdx
-2-> 408e41: 77 43 ja 408e86 <Main_main1_info+0x9e>
408e43: 48 8d 5a fa lea -0x6(%rdx),%rbx
408e47: 48 ff c1 inc %rcx
408e4a: eb b5 jmp 408e01 <Main_main1_info+0x19>
408e4c: 0f 1f 40 00 nopl 0x0(%rax)
408e50: c2 00 00 retq $0x0
</pre>
<br>
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<pre class="moz-quote-pre" wrap="">_______________________________________________
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</pre>
</blockquote>
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