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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.
include AsmMacros.inc
;; 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
g_GCShadow TEXTEQU <?g_GCShadow@@3PEAEEA>
g_GCShadowEnd TEXTEQU <?g_GCShadowEnd@@3PEAEEA>
INVALIDGCVALUE EQU 0CCCCCCCDh
EXTERN g_GCShadow : QWORD
EXTERN g_GCShadowEnd : QWORD
UPDATE_GC_SHADOW macro BASENAME, REFREG, DESTREG
;; If g_GCShadow is 0, don't perform the check.
cmp g_GCShadow, 0
je &BASENAME&_UpdateShadowHeap_Done_&REFREG&
;; 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). Note that this is naughty since we're altering the stack pointer outside of
;; the prolog inside a method without a frame. But given that this is only debug code and generally we
;; shouldn't be walking the stack at this point it seems preferable to recoding the all the barrier
;; variants to set up frames. Unlike RhpBulkWriteBarrier below which is treated as a helper call using the
;; usual calling convention, the compiler knows exactly which registers are trashed in the simple write
;; barrier case, so we don't have any more scratch registers to play with (and doing so would only make
;; things harder if at a later stage we want to allow multiple barrier versions based on the input
;; registers).
push DESTREG
;; Transform DESTREG into the equivalent address in the shadow heap.
sub DESTREG, g_lowest_address
jb &BASENAME&_UpdateShadowHeap_PopThenDone_&REFREG&
add DESTREG, [g_GCShadow]
cmp DESTREG, [g_GCShadowEnd]
ja &BASENAME&_UpdateShadowHeap_PopThenDone_&REFREG&
;; Update the shadow heap.
mov [DESTREG], REFREG
;; Now check that the real heap location still contains the value we just wrote into the shadow heap. This
;; read must be strongly ordered wrt to the previous write to prevent race conditions. We also need to
;; recover the old value of DESTREG for the comparison so use an xchg instruction (which has an implicit lock
;; prefix).
xchg [rsp], DESTREG
cmp [DESTREG], REFREG
jne &BASENAME&_UpdateShadowHeap_Invalidate_&REFREG&
;; The original DESTREG value is now restored but the stack has a value (the shadow version of the
;; location) pushed. Need to discard this push before we are done.
add rsp, 8
jmp &BASENAME&_UpdateShadowHeap_Done_&REFREG&
&BASENAME&_UpdateShadowHeap_Invalidate_&REFREG&:
;; Someone went and updated the real heap. We need to invalidate the shadow location since we can't
;; guarantee whose shadow update won.
;; Retrieve shadow location from the stack and restore original DESTREG to the stack. This is an
;; additional memory barrier we don't require but it's on the rare path and x86 doesn't have an xchg
;; variant that doesn't implicitly specify the lock prefix. Note that INVALIDGCVALUE is a 64-bit
;; immediate and therefore must be moved into a register before it can be written to the shadow
;; location.
xchg [rsp], DESTREG
push REFREG
mov REFREG, INVALIDGCVALUE
mov qword ptr [DESTREG], REFREG
pop REFREG
&BASENAME&_UpdateShadowHeap_PopThenDone_&REFREG&:
;; Restore original DESTREG value from the stack.
pop DESTREG
&BASENAME&_UpdateShadowHeap_Done_&REFREG&:
endm
else ; WRITE_BARRIER_CHECK
UPDATE_GC_SHADOW macro BASENAME, REFREG, DESTREG
endm
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_UNCHECKED_WRITE_BARRIER_CORE macro BASENAME, 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 BASENAME, REFREG, rcx
;; If the reference is to an object that's not in an ephemeral generation we have no need to track it
;; (since the object won't be collected or moved by an ephemeral collection).
cmp REFREG, [g_ephemeral_low]
jb &BASENAME&_NoBarrierRequired_&REFREG&
cmp REFREG, [g_ephemeral_high]
jae &BASENAME&_NoBarrierRequired_&REFREG&
;; We have a location on the GC heap being updated with a reference to an ephemeral object so we must
;; track this write. The location address is translated into an offset in the card table bitmap. We set
;; an entire byte in the card table since it's quicker than messing around with bitmasks and we only write
;; the byte if it hasn't already been done since writes are expensive and impact scaling.
shr rcx, 11
add rcx, [g_card_table]
cmp byte ptr [rcx], 0FFh
jne &BASENAME&_UpdateCardTable_&REFREG&
&BASENAME&_NoBarrierRequired_&REFREG&:
ret
;; We get here if it's necessary to update the card table.
&BASENAME&_UpdateCardTable_&REFREG&:
mov byte ptr [rcx], 0FFh
ret
endm
;; 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. One argument is taken, the
;; name of the register that will hold the object reference (this should be in upper case as it's used in the
;; definition of the name of the helper).
DEFINE_UNCHECKED_WRITE_BARRIER macro REFREG, EXPORT_REG_NAME
;; Define a helper with a name of the form RhpAssignRefEAX etc. (along with suitable calling standard
;; decoration). The location to be updated is in DESTREG. The object reference that will be assigned into that
;; location is in one of the other general registers determined by the value of REFREG.
;; 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 on the first instruction
;; - Function "UnwindWriteBarrierToCaller" assumes the stack contains just the pushed return address
LEAF_ENTRY RhpAssignRef&EXPORT_REG_NAME&, _TEXT
;; Export the canonical write barrier under unqualified name as well
ifidni <REFREG>, <RDX>
ALTERNATE_ENTRY RhpAssignRef
ALTERNATE_ENTRY RhpAssignRefAVLocation
endif
;; Write the reference into the location. Note that we rely on the fact that no GC can occur between here
;; and the card table update we may perform below.
mov qword ptr [rcx], REFREG
DEFINE_UNCHECKED_WRITE_BARRIER_CORE RhpAssignRef, REFREG
LEAF_END RhpAssignRef&EXPORT_REG_NAME&, _TEXT
endm
;; One day we might have write barriers for all the possible argument registers but for now we have
;; just one write barrier that assumes the input register is RDX.
DEFINE_UNCHECKED_WRITE_BARRIER RDX, EDX
;;
;; 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.
;;
DEFINE_CHECKED_WRITE_BARRIER_CORE macro BASENAME, REFREG
;; The location being updated might not even lie in the GC heap (a handle or stack location for instance),
;; in which case no write barrier is required.
cmp rcx, [g_lowest_address]
jb &BASENAME&_NoBarrierRequired_&REFREG&
cmp rcx, [g_highest_address]
jae &BASENAME&_NoBarrierRequired_&REFREG&
DEFINE_UNCHECKED_WRITE_BARRIER_CORE BASENAME, REFREG
endm
;; 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. One argument is taken, the
;; name of the register that will hold the object reference (this should be in upper case as it's used in the
;; definition of the name of the helper).
DEFINE_CHECKED_WRITE_BARRIER macro REFREG, EXPORT_REG_NAME
;; Define a helper with a name of the form RhpCheckedAssignRefEAX etc. (along with suitable calling standard
;; decoration). The location to be updated is always in RCX. The object reference that will be assigned into
;; that location is in one of the other general registers determined by the value of REFREG.
;; 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 on the first instruction
;; - Function "UnwindWriteBarrierToCaller" assumes the stack contains just the pushed return address
LEAF_ENTRY RhpCheckedAssignRef&EXPORT_REG_NAME&, _TEXT
;; Export the canonical write barrier under unqualified name as well
ifidni <REFREG>, <RDX>
ALTERNATE_ENTRY RhpCheckedAssignRef
ALTERNATE_ENTRY RhpCheckedAssignRefAVLocation
endif
;; Write the reference into the location. Note that we rely on the fact that no GC can occur between here
;; and the card table update we may perform below.
mov qword ptr [rcx], REFREG
DEFINE_CHECKED_WRITE_BARRIER_CORE RhpCheckedAssignRef, REFREG
LEAF_END RhpCheckedAssignRef&EXPORT_REG_NAME&, _TEXT
endm
;; One day we might have write barriers for all the possible argument registers but for now we have
;; just one write barrier that assumes the input register is RDX.
DEFINE_CHECKED_WRITE_BARRIER RDX, EDX
;; 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 the stack contains just the pushed return address
LEAF_ENTRY RhpCheckedLockCmpXchg, _TEXT
mov rax, r8
ALTERNATE_ENTRY RhpCheckedLockCmpXchgAVLocation
lock cmpxchg [rcx], rdx
jne RhpCheckedLockCmpXchg_NoBarrierRequired_RDX
DEFINE_CHECKED_WRITE_BARRIER_CORE RhpCheckedLockCmpXchg, RDX
LEAF_END RhpCheckedLockCmpXchg, _TEXT
;; 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 RhpCheckedXchgAVLocation
;; - Function "UnwindWriteBarrierToCaller" assumes the stack contains just the pushed return address
LEAF_ENTRY RhpCheckedXchg, _TEXT
;; Setup rax with the new object for the exchange, that way it will automatically hold the correct result
;; afterwards and we can leave rdx unaltered ready for the GC write barrier below.
mov rax, rdx
ALTERNATE_ENTRY RhpCheckedXchgAVLocation
xchg [rcx], rax
DEFINE_CHECKED_WRITE_BARRIER_CORE RhpCheckedXchg, RDX
LEAF_END RhpCheckedXchg, _TEXT
;;
;; RhpByRefAssignRef simulates movs instruction for object references.
;;
;; On entry:
;; rdi: address of ref-field (assigned to)
;; rsi: address of the data (source)
;; rcx: be trashed
;;
;; On exit:
;; rdi, rsi are incremented by 8,
;; rcx: trashed
;;
LEAF_ENTRY RhpByRefAssignRef, _TEXT
mov rcx, [rsi]
mov [rdi], rcx
;; Check whether the writes were even into the heap. If not there's no card update required.
cmp rdi, [g_lowest_address]
jb RhpByRefAssignRef_NotInHeap
cmp rdi, [g_highest_address]
jae RhpByRefAssignRef_NotInHeap
;; 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 BASENAME, rcx, rdi
;; If the reference is to an object that's not in an ephemeral generation we have no need to track it
;; (since the object won't be collected or moved by an ephemeral collection).
cmp rcx, [g_ephemeral_low]
jb RhpByRefAssignRef_NotInHeap
cmp rcx, [g_ephemeral_high]
jae RhpByRefAssignRef_NotInHeap
;; move current rdi value into rcx and then increment the pointers
mov rcx, rdi
add rsi, 8h
add rdi, 8h
;; We have a location on the GC heap being updated with a reference to an ephemeral object so we must
;; track this write. The location address is translated into an offset in the card table bitmap. We set
;; an entire byte in the card table since it's quicker than messing around with bitmasks and we only write
;; the byte if it hasn't already been done since writes are expensive and impact scaling.
shr rcx, 11
add rcx, [g_card_table]
cmp byte ptr [rcx], 0FFh
jne RhpByRefAssignRef_UpdateCardTable
ret
;; We get here if it's necessary to update the card table.
RhpByRefAssignRef_UpdateCardTable:
mov byte ptr [rcx], 0FFh
ret
RhpByRefAssignRef_NotInHeap:
; Increment the pointers before leaving
add rdi, 8h
add rsi, 8h
ret
LEAF_END RhpByRefAssignRef, _TEXT
end
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