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;; Licensed to the .NET Foundation under one or more agreements.
;; The .NET Foundation licenses this file to you under the MIT license.
;;
;; Define the helpers used to implement the write barrier required when writing an object reference into a
;; location residing on the GC heap. Such write barriers allow the GC to optimize which objects in
;; non-ephemeral generations need to be scanned for references to ephemeral objects during an ephemeral
;; collection.
;;
#include "AsmMacros.h"
TEXTAREA
;; Macro used to copy contents of newly updated GC heap locations to a shadow copy of the heap. This is used
;; during garbage collections to verify that object references where never written to the heap without using a
;; write barrier. Note that we're potentially racing to update the shadow heap while other threads are writing
;; new references to the real heap. Since this can't be solved perfectly without critical sections around the
;; entire update process, we instead update the shadow location and then re-check the real location (as two
;; ordered operations) and if there is a disparity we'll re-write the shadow location with a special value
;; (INVALIDGCVALUE) which disables the check for that location. Since the shadow heap is only validated at GC
;; time and these write barrier operations are atomic wrt to GCs this is sufficient to guarantee that the
;; shadow heap contains only valid copies of real heap values or INVALIDGCVALUE.
#ifdef WRITE_BARRIER_CHECK
SETALIAS g_GCShadow, ?g_GCShadow@@3PEAEEA
SETALIAS g_GCShadowEnd, ?g_GCShadowEnd@@3PEAEEA
EXTERN $g_GCShadow
EXTERN $g_GCShadowEnd
INVALIDGCVALUE EQU 0xCCCCCCCD
MACRO
;; On entry:
;; $destReg: location to be updated (cannot be x12,x17)
;; $refReg: objectref to be stored (cannot be x12,x17)
;;
;; On exit:
;; x12,x17: trashed
;; other registers are preserved
;;
UPDATE_GC_SHADOW $destReg, $refReg
;; If g_GCShadow is 0, don't perform the check.
PREPARE_EXTERNAL_VAR_INDIRECT $g_GCShadow, x12
cbz x12, %ft1
;; Save $destReg since we're about to modify it (and we need the original value both within the macro and
;; once we exit the macro).
mov x17, $destReg
;; Transform $destReg into the equivalent address in the shadow heap.
PREPARE_EXTERNAL_VAR_INDIRECT g_lowest_address, x12
subs $destReg, $destReg, x12
blo %ft0
PREPARE_EXTERNAL_VAR_INDIRECT $g_GCShadow, x12
add $destReg, $destReg, x12
PREPARE_EXTERNAL_VAR_INDIRECT g_GCShadowEnd, x12
cmp $destReg, x12
bhs %ft0
;; Update the shadow heap.
str $refReg, [$destReg]
;; The following read must be strongly ordered wrt to the write we've just performed in order to
;; prevent race conditions.
dmb ish
;; Now check that the real heap location still contains the value we just wrote into the shadow heap.
mov x12, x17
ldr x12, [x12]
cmp x12, $refReg
beq %ft0
;; Someone went and updated the real heap. We need to invalidate the shadow location since we can't
;; guarantee whose shadow update won.
MOVL64 x12, INVALIDGCVALUE, 0
str x12, [$destReg]
0
;; Restore original $destReg value
mov $destReg, x17
1
MEND
#else // WRITE_BARRIER_CHECK
MACRO
UPDATE_GC_SHADOW $destReg, $refReg
MEND
#endif // WRITE_BARRIER_CHECK
;; There are several different helpers used depending on which register holds the object reference. Since all
;; the helpers have identical structure we use a macro to define this structure. Two arguments are taken, the
;; name of the register that points to the location to be updated and the name of the register that holds the
;; object reference (this should be in upper case as it's used in the definition of the name of the helper).
;; Define a sub-macro first that expands to the majority of the barrier implementation. This is used below for
;; some interlocked helpers that need an inline barrier.
MACRO
;; On entry:
;; $destReg: location to be updated (cannot be x12,x17)
;; $refReg: objectref to be stored (cannot be x12,x17)
;;
;; On exit:
;; x12,x17: trashed
;;
INSERT_UNCHECKED_WRITE_BARRIER_CORE $destReg, $refReg
;; Update the shadow copy of the heap with the same value just written to the same heap. (A no-op unless
;; we're in a debug build and write barrier checking has been enabled).
UPDATE_GC_SHADOW $destReg, $refReg
#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
// Update the write watch table if necessary
PREPARE_EXTERNAL_VAR_INDIRECT g_write_watch_table, x12
cbz x12, %ft2
add x12, x12, $destReg, lsr #0xc // SoftwareWriteWatch::AddressToTableByteIndexShift
ldrb w17, [x12]
cbnz x17, %ft2
mov w17, #0xFF
strb w17, [x12]
#endif
2
;; We can skip the card table write if the reference is to
;; an object not on the epehemeral segment.
PREPARE_EXTERNAL_VAR_INDIRECT g_ephemeral_low, x12
cmp $refReg, x12
blo %ft0
PREPARE_EXTERNAL_VAR_INDIRECT g_ephemeral_high, x12
cmp $refReg, x12
bhs %ft0
;; Set this object's card, if it hasn't already been set.
PREPARE_EXTERNAL_VAR_INDIRECT g_card_table, x12
add x17, x12, $destReg lsr #11
;; Check that this card hasn't already been written. Avoiding useless writes is a big win on
;; multi-proc systems since it avoids cache trashing.
ldrb w12, [x17]
cmp x12, 0xFF
beq %ft0
mov x12, 0xFF
strb w12, [x17]
#ifdef FEATURE_MANUALLY_MANAGED_CARD_BUNDLES
// Check if we need to update the card bundle table
PREPARE_EXTERNAL_VAR_INDIRECT g_card_bundle_table, x12
add x17, x12, $destReg, lsr #21
ldrb w12, [x17]
cmp x12, 0xFF
beq %ft0
mov x12, 0xFF
strb w12, [x17]
#endif
0
;; Exit label
MEND
MACRO
;; On entry:
;; $destReg: location to be updated (cannot be x12,x17)
;; $refReg: objectref to be stored (cannot be x12,x17)
;;
;; On exit:
;; x12, x17: trashed
;;
INSERT_CHECKED_WRITE_BARRIER_CORE $destReg, $refReg
;; The "check" of this checked write barrier - is $destReg
;; within the heap? if no, early out.
PREPARE_EXTERNAL_VAR_INDIRECT g_lowest_address, x12
cmp $destReg, x12
PREPARE_EXTERNAL_VAR_INDIRECT g_highest_address, x12
;; If $destReg >= g_lowest_address, compare $destReg to g_highest_address.
;; Otherwise, set the C flag (0x2) to take the next branch.
ccmp $destReg, x12, #0x2, hs
bhs %ft0
INSERT_UNCHECKED_WRITE_BARRIER_CORE $destReg, $refReg
0
;; Exit label
MEND
;; void JIT_ByRefWriteBarrier
;; On entry:
;; x13 : the source address (points to object reference to write)
;; x14 : the destination address (object reference written here)
;;
;; On exit:
;; x13 : incremented by 8
;; x14 : incremented by 8
;; x15 : trashed
;; x12, x17 : trashed
;;
LEAF_ENTRY RhpByRefAssignRefArm64, _TEXT
ldr x15, [x13], 8
b RhpCheckedAssignRefArm64
LEAF_END RhpByRefAssignRefArm64
;; JIT_CheckedWriteBarrier(Object** dst, Object* src)
;;
;; Write barrier for writes to objects that may reside
;; on the managed heap.
;;
;; On entry:
;; x14 : the destination address (LHS of the assignment).
;; May not be a heap location (hence the checked).
;; x15 : the object reference (RHS of the assignment)
;;
;; On exit:
;; x12, x17 : trashed
;; x14 : incremented by 8
LEAF_ENTRY RhpCheckedAssignRefArm64
;; is destReg within the heap?
PREPARE_EXTERNAL_VAR_INDIRECT g_lowest_address, x12
cmp x14, x12
PREPARE_EXTERNAL_VAR_INDIRECT g_highest_address, x12
ccmp x14, x12, #0x2, hs
blo RhpAssignRefArm64
NotInHeap
ALTERNATE_ENTRY RhpCheckedAssignRefAVLocation
str x15, [x14], 8
ret
LEAF_END RhpCheckedAssignRefArm64
;; JIT_WriteBarrier(Object** dst, Object* src)
;;
;; Write barrier for writes to objects that are known to
;; reside on the managed heap.
;;
;; On entry:
;; x14 : the destination address (LHS of the assignment)
;; x15 : the object reference (RHS of the assignment)
;;
;; On exit:
;; x12, x17 : trashed
;; x14 : incremented by 8
LEAF_ENTRY RhpAssignRefArm64
ALTERNATE_ENTRY RhpAssignRefAVLocation
stlr x15, [x14]
INSERT_UNCHECKED_WRITE_BARRIER_CORE x14, x15
add x14, x14, 8
ret
LEAF_END RhpAssignRefArm64
;; same as RhpAssignRefArm64, but with standard ABI.
LEAF_ENTRY RhpAssignRef
mov x14, x0 ; x14 = dst
mov x15, x1 ; x15 = val
b RhpAssignRefArm64
LEAF_END RhpAssignRef
;; Interlocked operation helpers where the location is an objectref, thus requiring a GC write barrier upon
;; successful updates.
;; WARNING: Code in EHHelpers.cpp makes assumptions about write barrier code, in particular:
;; - Function "InWriteBarrierHelper" assumes an AV due to passed in null pointer will happen at RhpCheckedLockCmpXchgAVLocation
;; - Function "UnwindSimpleHelperToCaller" assumes no registers were pushed and LR contains the return address
;; RhpCheckedLockCmpXchg(Object** dest, Object* value, Object* comparand)
;;
;; Interlocked compare exchange on objectref.
;;
;; On entry:
;; x0 : pointer to objectref
;; x1 : exchange value
;; x2 : comparand
;;
;; On exit:
;; x0: original value of objectref
;; x10, x12, x17: trashed
;;
LEAF_ENTRY RhpCheckedLockCmpXchg
CmpXchgRetry
;; Check location value is what we expect.
ALTERNATE_ENTRY RhpCheckedLockCmpXchgAVLocation
ldaxr x10, [x0]
cmp x10, x2
bne CmpXchgNoUpdate
;; Current value matches comparand, attempt to update with the new value.
stlxr w12, x1, [x0]
cbnz w12, CmpXchgRetry
;; We've successfully updated the value of the objectref so now we need a GC write barrier.
;; The following barrier code takes the destination in x0 and the value in x1 so the arguments are
;; already correctly set up.
INSERT_CHECKED_WRITE_BARRIER_CORE x0, x1
CmpXchgNoUpdate
;; x10 still contains the original value.
mov x0, x10
InterlockedOperationBarrier
ret lr
LEAF_END RhpCheckedLockCmpXchg
;; WARNING: Code in EHHelpers.cpp makes assumptions about write barrier code, in particular:
;; - Function "InWriteBarrierHelper" assumes an AV due to passed in null pointer will happen within at RhpCheckedXchgAVLocation
;; - Function "UnwindSimpleHelperToCaller" assumes no registers were pushed and LR contains the return address
;; RhpCheckedXchg(Object** destination, Object* value)
;;
;; Interlocked exchange on objectref.
;;
;; On entry:
;; x0 : pointer to objectref
;; x1 : exchange value
;;
;; On exit:
;; x0: original value of objectref
;; x10: trashed
;; x12, x17: trashed
;;
LEAF_ENTRY RhpCheckedXchg
ExchangeRetry
;; Read the existing memory location.
ALTERNATE_ENTRY RhpCheckedXchgAVLocation
ldaxr x10, [x0]
;; Attempt to update with the new value.
stlxr w12, x1, [x0]
cbnz w12, ExchangeRetry
;; We've successfully updated the value of the objectref so now we need a GC write barrier.
;; The following barrier code takes the destination in x0 and the value in x1 so the arguments are
;; already correctly set up.
INSERT_CHECKED_WRITE_BARRIER_CORE x0, x1
;; x10 still contains the original value.
mov x0, x10
InterlockedOperationBarrier
ret
LEAF_END RhpCheckedXchg
end
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