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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.
;; See the LICENSE file in the project root for more information.
;;
;; 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
;; $refReg: objectref to be stored
;;
;; On exit:
;; x9,x10: trashed
;; other registers are preserved
;;
UPDATE_GC_SHADOW $destReg, $refReg
;; If g_GCShadow is 0, don't perform the check.
adrp x9, $g_GCShadow
ldr x9, [x9, $g_GCShadow]
cbz x9, %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 x10, $destReg
;; Transform $destReg into the equivalent address in the shadow heap.
adrp x9, g_lowest_address
ldr x9, [x9, g_lowest_address]
subs $destReg, $destReg, x9
blt %ft0
adrp x9, $g_GCShadow
ldr x9, [x9, $g_GCShadow]
add $destReg, $destReg, x9
adrp x9, $g_GCShadowEnd
ldr x9, [x9, $g_GCShadowEnd]
cmp $destReg, x9
bgt %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 x9, x10
ldr x9, [x9]
cmp x9, $refReg
beq %ft0
;; Someone went and updated the real heap. We need to invalidate the shadow location since we can't
;; guarantee whose shadow update won.
MOVL64 x9, INVALIDGCVALUE, 0
str x9, [$destReg]
0
;; Restore original $destReg value
mov $destReg, x10
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
;; $refReg: objectref to be stored
;;
;; On exit:
;; $destReg: trashed
;; x9: 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
;; We can skip the card table write if the reference is to
;; an object not on the epehemeral segment.
adrp x9, g_ephemeral_low
ldr x9, [x9, g_ephemeral_low]
cmp $refReg, x9
blt %ft0
adrp x9, g_ephemeral_high
ldr x9, [x9, g_ephemeral_high]
cmp $refReg, x9
bge %ft0
;; Set this object's card, if it hasn't already been set.
adrp x9, g_card_table
ldr x9, [x9, g_card_table]
add $destReg, x9, $destReg lsr #11
;; Check that this card hasn't already been written. Avoiding useless writes is a big win on
;; multi-proc systems since it avoids cache thrashing.
ldrb w9, [$destReg]
cmp x9, 0xFF
beq %ft0
mov x9, 0xFF
strb w9, [$destReg]
0
;; Exit label
MEND
MACRO
;; On entry:
;; $destReg: location to be updated
;; $refReg: objectref to be stored
;;
;; On exit:
;; $destReg: trashed
;; x9: trashed
;;
INSERT_CHECKED_WRITE_BARRIER_CORE $destReg, $refReg
;; The "check" of this checked write barrier - is $destReg
;; within the heap? if no, early out.
adrp x9, g_lowest_address
ldr x9, [x9, g_lowest_address]
cmp $destReg, x9
blt %ft0
adrp x9, g_highest_address
ldr x9, [x9, g_highest_address]
cmp $destReg, x9
bgt %ft0
INSERT_UNCHECKED_WRITE_BARRIER_CORE $destReg, $refReg
0
;; Exit label
MEND
;; RhpCheckedAssignRef(Object** dst, Object* src)
;;
;; Write barrier for writes to objects that may reside
;; on the managed heap.
;;
;; On entry:
;; x0 : the destination address (LHS of the assignment).
;; May not be an object reference (hence the checked).
;; x1 : the object reference (RHS of the assignment).
;; On exit:
;; x1 : trashed
;; x9 : trashed
LEAF_ENTRY RhpCheckedAssignRef
ALTERNATE_ENTRY RhpCheckedAssignRefAVLocation
ALTERNATE_ENTRY RhpCheckedAssignRefX1
ALTERNATE_ENTRY RhpCheckedAssignRefX1AVLocation
stlr x1, [x0]
INSERT_CHECKED_WRITE_BARRIER_CORE x0, x1
ret
LEAF_END RhpCheckedAssignRef
;; RhpAssignRef(Object** dst, Object* src)
;;
;; Write barrier for writes to objects that are known to
;; reside on the managed heap.
;;
;; On entry:
;; x0 : the destination address (LHS of the assignment).
;; x1 : the object reference (RHS of the assignment).
;; On exit:
;; x1 : trashed
;; x9 : trashed
LEAF_ENTRY RhpAssignRef
ALTERNATE_ENTRY RhpAssignRefAVLocation
ALTERNATE_ENTRY RhpAssignRefX1
ALTERNATE_ENTRY RhpAssignRefX1AVLocation
stlr x1, [x0]
INSERT_UNCHECKED_WRITE_BARRIER_CORE x0, x1
ret
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 "UnwindWriteBarrierToCaller" assumes no registers where 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
;; x9: trashed
;; x10: trashed
;;
LEAF_ENTRY RhpCheckedLockCmpXchg
ALTERNATE_ENTRY RhpCheckedLockCmpXchgAVLocation
CmpXchgRetry
;; Check location value is what we expect.
ldaxr x10, [x0]
cmp x10, x2
bne CmpXchgNoUpdate
;; Current value matches comparand, attempt to update with the new value.
stlxr w9, x1, [x0]
cbnz w9, 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
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 "UnwindWriteBarrierToCaller" assumes no registers where 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
;; x9: trashed
;; x10: trashed
;;
LEAF_ENTRY RhpCheckedXchg
ALTERNATE_ENTRY RhpCheckedXchgAVLocation
ExchangeRetry
;; Read the existing memory location.
ldaxr x10, [x0]
;; Attempt to update with the new value.
stlxr w9, x1, [x0]
cbnz w9, 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
ret
LEAF_END RhpCheckedXchg
end
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