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Ensure memory ordering in LazyWithContext for ARM64 #19271

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Merged
T-Gro merged 3 commits into from
Feb 11, 2026
Merged

Ensure memory ordering in LazyWithContext for ARM64 #19271

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bc89f8e
Ensure memory ordering in LazyWithContext for ARM64
majocha Feb 11, 2026
22b17e4
Merge branch 'main' into lazywithcontext-arm
T-Gro Feb 11, 2026
7a6ac47
Merge branch 'main' into lazywithcontext-arm
majocha Feb 11, 2026
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12 changes: 11 additions & 1 deletion src/Compiler/Utilities/illib.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -1044,7 +1044,12 @@ type LazyWithContext<'T, 'Ctxt> =

member x.Force(ctxt: 'Ctxt) =
match x.funcOrException with
| null -> x.value
| null ->
// Ensure we read the latest value after seeing funcOrException as null.
// On weakly-ordered architectures like ARM64, without this barrier the CPU
// can reorder the reads and we may observe a stale (default/null) x.value.
Thread.MemoryBarrier()
x.value
| _ ->
// Enter the lock in case another thread is in the process of evaluating the result
Monitor.Enter x
Expand All @@ -1066,6 +1071,11 @@ type LazyWithContext<'T, 'Ctxt> =
try
let res = f ctxt
x.value <- res
// Ensure the write to x.value is globally visible before funcOrException
// is set to null, so that readers on the fast path of Force see the result.
// Without this barrier, ARM64 may reorder the stores causing readers to see
// funcOrException as null but x.value as stale (Unchecked.defaultof).
Thread.MemoryBarrier()
x.funcOrException <- null
res
with RecoverableException exn ->
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