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WriteBarriers.S
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WriteBarriers.S
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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.
.intel_syntax noprefix
#include <unixasmmacros.inc>
#ifdef WRITE_BARRIER_CHECK
.macro UPDATE_GC_SHADOW BASENAME, REFREG, DESTREG
// If g_GCShadow is 0, don't perform the check.
cmp qword ptr [C_VAR(g_GCShadow)], 0
je LOCAL_LABEL(\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. 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, [C_VAR(g_lowest_address)]
jb LOCAL_LABEL(\BASENAME\()_UpdateShadowHeap_PopThenDone_\REFREG)
add \DESTREG, [C_VAR(g_GCShadow)]
cmp \DESTREG, [C_VAR(g_GCShadowEnd)]
jae LOCAL_LABEL(\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 LOCAL_LABEL(\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 LOCAL_LABEL(\BASENAME\()_UpdateShadowHeap_Done_\REFREG)
LOCAL_LABEL(\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
movabs \REFREG, INVALIDGCVALUE
mov qword ptr [\DESTREG], \REFREG
pop \REFREG
LOCAL_LABEL(\BASENAME\()_UpdateShadowHeap_PopThenDone_\REFREG):
// Restore original DESTREG value from the stack.
pop \DESTREG
LOCAL_LABEL(\BASENAME\()_UpdateShadowHeap_Done_\REFREG):
.endm
#else // WRITE_BARRIER_CHECK
.macro UPDATE_GC_SHADOW 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).
.macro DEFINE_UNCHECKED_WRITE_BARRIER_CORE 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, rdi
#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
mov r11, [C_VAR(g_write_watch_table)]
cmp r11, 0x0
je LOCAL_LABEL(\BASENAME\()_CheckCardTable_\REFREG)
mov r10, rdi
shr r10, 0xC // SoftwareWriteWatch::AddressToTableByteIndexShift
add r10, r11
cmp byte ptr [r10], 0x0
jne LOCAL_LABEL(\BASENAME\()_CheckCardTable_\REFREG)
mov byte ptr [r10], 0xFF
#endif
LOCAL_LABEL(\BASENAME\()_CheckCardTable_\REFREG):
// 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, [C_VAR(g_ephemeral_low)]
jb LOCAL_LABEL(\BASENAME\()_NoBarrierRequired_\REFREG)
cmp \REFREG, [C_VAR(g_ephemeral_high)]
jae LOCAL_LABEL(\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 rdi, 0x0B
mov r10, [C_VAR(g_card_table)]
cmp byte ptr [rdi + r10], 0x0FF
je LOCAL_LABEL(\BASENAME\()_NoBarrierRequired_\REFREG)
// We get here if it's necessary to update the card table.
mov byte ptr [rdi + r10], 0xFF
#ifdef FEATURE_MANUALLY_MANAGED_CARD_BUNDLES
// Shift rdi by 0x0A more to get the card bundle byte (we shifted by 0x0B already)
shr rdi, 0x0A
add rdi, [C_VAR(g_card_bundle_table)]
cmp byte ptr [rdi], 0xFF
je LOCAL_LABEL(\BASENAME\()_NoBarrierRequired_\REFREG)
mov byte ptr [rdi], 0xFF
#endif
LOCAL_LABEL(\BASENAME\()_NoBarrierRequired_\REFREG):
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).
.macro DEFINE_UNCHECKED_WRITE_BARRIER 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 "UnwindSimpleHelperToCaller" 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
.ifc \REFREG, RSI
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 [rdi], \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 RSI.
DEFINE_UNCHECKED_WRITE_BARRIER RSI, ESI
//
// 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.
//
.macro DEFINE_CHECKED_WRITE_BARRIER_CORE 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 rdi, [C_VAR(g_lowest_address)]
jb LOCAL_LABEL(\BASENAME\()_NoBarrierRequired_\REFREG)
cmp rdi, [C_VAR(g_highest_address)]
jae LOCAL_LABEL(\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).
.macro DEFINE_CHECKED_WRITE_BARRIER 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 RDI. 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 "UnwindSimpleHelperToCaller" 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
.ifc \REFREG, RSI
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 [rdi], \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 RSI.
DEFINE_CHECKED_WRITE_BARRIER RSI, ESI
// 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 the stack contains just the pushed return address
LEAF_ENTRY RhpCheckedLockCmpXchg, _TEXT
mov rax, rdx
ALTERNATE_ENTRY RhpCheckedLockCmpXchgAVLocation
lock cmpxchg [rdi], rsi
jne LOCAL_LABEL(RhpCheckedLockCmpXchg_NoBarrierRequired_RSI)
DEFINE_CHECKED_WRITE_BARRIER_CORE RhpCheckedLockCmpXchg, RSI
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 "UnwindSimpleHelperToCaller" 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, rsi
ALTERNATE_ENTRY RhpCheckedXchgAVLocation
xchg [rdi], rax
DEFINE_CHECKED_WRITE_BARRIER_CORE RhpCheckedXchg, RSI
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, [C_VAR(g_lowest_address)]
jb LOCAL_LABEL(RhpByRefAssignRef_NotInHeap)
cmp rdi, [C_VAR(g_highest_address)]
jae LOCAL_LABEL(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
#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
mov r11, [C_VAR(g_write_watch_table)]
cmp r11, 0x0
je LOCAL_LABEL(RhpByRefAssignRef_CheckCardTable)
mov r10, rdi
shr r10, 0xC // SoftwareWriteWatch::AddressToTableByteIndexShift
add r10, r11
cmp byte ptr [r10], 0x0
jne LOCAL_LABEL(RhpByRefAssignRef_CheckCardTable)
mov byte ptr [r10], 0xFF
#endif
LOCAL_LABEL(RhpByRefAssignRef_CheckCardTable):
// 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, [C_VAR(g_ephemeral_low)]
jb LOCAL_LABEL(RhpByRefAssignRef_NotInHeap)
cmp rcx, [C_VAR(g_ephemeral_high)]
jae LOCAL_LABEL(RhpByRefAssignRef_NotInHeap)
// move current rdi value into rcx, we need to keep rdi and eventually increment by 8
mov rcx, rdi
// 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, 0x0B
mov r10, [C_VAR(g_card_table)]
cmp byte ptr [rcx + r10], 0x0FF
je LOCAL_LABEL(RhpByRefAssignRef_NotInHeap)
// We get here if it's necessary to update the card table.
mov byte ptr [rcx + r10], 0xFF
#ifdef FEATURE_MANUALLY_MANAGED_CARD_BUNDLES
// Shift rcx by 0x0A more to get the card bundle byte (we shifted by 0x0B already)
shr rcx, 0x0A
add rcx, [C_VAR(g_card_bundle_table)]
cmp byte ptr [rcx], 0xFF
je LOCAL_LABEL(RhpByRefAssignRef_NotInHeap)
mov byte ptr [rcx], 0xFF
#endif
LOCAL_LABEL(RhpByRefAssignRef_NotInHeap):
// Increment the pointers before leaving
add rdi, 0x8
add rsi, 0x8
ret
LEAF_END RhpByRefAssignRef, _TEXT