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Minor fixes to write barriers #75478

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Merged
VSadov merged 2 commits into from
Apr 11, 2023
Merged

Minor fixes to write barriers #75478

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cb1509d
Minor fixes to write barriers
AntonLapounov Sep 12, 2022
da811d6
use the same predicated compare as on Unix when checking ephemeral lo…
VSadov Apr 10, 2023
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use the same predicated compare as on Unix when checking ephemeral lo… 
…w/high
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@VSadov
VSadov committed Apr 10, 2023
commit da811d6121b60f45f3c211df626ed12af34df9a4
9 changes: 7 additions & 2 deletions src/coreclr/vm/arm64/asmhelpers.asm
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Expand Up @@ -476,10 +476,15 @@ CheckCardTable
ldr x12, wbs_ephemeral_low
cbz x12, SkipEphemeralCheck
cmp x15, x12
blo Exit

ldr x12, wbs_ephemeral_high
cmp x15, x12

; Compare against the upper bound if the previous comparison indicated
; that the destination address is greater than or equal to the lower
; bound. Otherwise, set the C flag (specified by the 0x2) so that the
; branch to exit is taken.
ccmp x15, x12, #0x2, hs

bhs Exit
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@VSadov VSadov Sep 14, 2022

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The .S version is using ccmp here. I think we should use the same pattern in both cases.

I wonder which would be better though. In theory the ephemeral check should more often fail than pass, statistically. In a large heap most objects are tenured. Ephemeral set is supposed to be small.

On the other hand both patterns probably work the same on a speculative CPU, but the predicated pattern is shorter by one instruction.

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@VSadov VSadov Sep 14, 2022

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And on server GC ephemeral check will always pass. I think, since we are touching this, it should be switched to the predicated form like in .S


SkipEphemeralCheck
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