T0b1/wasmtime
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Support heaps with no offset-guard pages.

Browse Source 
Also, say "guard-offset pages" rather than just "guard pages" to describe the
region of a heap which is never accessible and which exists to support
optimizations for heap accesses with offsets.

And, introduce a `Uimm64` immediate type, and make all heap fields use
`Uimm64` instead of `Imm64` since they really are unsigned.
This commit is contained in:
Dan Gohman
2018-11-29 04:53:30 -08:00
parent 93696a80bb
commit a20c852148
27 changed files with 302 additions and 172 deletions
Download Patch File Download Diff File Expand all files Collapse all files

37
cranelift/docs/ir.rst
View File

@@ -651,7 +651,7 @@ architecture.
fontsize=10,
fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans"
]
"static" [label="mapped\npages|unmapped\npages|guard\npages"]
"static" [label="mapped\npages|unmapped\npages|offset_guard\npages"]
A heap appears as three consecutive ranges of address space:
@@ -661,9 +661,9 @@ A heap appears as three consecutive ranges of address space:
2. The *unmapped pages* is a possibly empty range of address space that may be
mapped in the future when the heap is grown. They are :term:`addressable` but
not :term:`accessible`.
3. The *guard pages* is a range of address space that is guaranteed to cause a
trap when accessed. It is used to optimize bounds checking for heap accesses
with a shared base pointer. They are :term:`addressable` but
3. The *offset-guard pages* is a range of address space that is guaranteed to
always cause a trap when accessed. It is used to optimize bounds checking for
heap accesses with a shared base pointer. They are :term:`addressable` but
not :term:`accessible`.
The *heap bound* is the total size of the mapped and unmapped pages. This is
@@ -683,10 +683,10 @@ A *static heap* starts out with all the address space it will ever need, so it
never moves to a different address. At the base address is a number of mapped
pages corresponding to the heap's current size. Then follows a number of
unmapped pages where the heap can grow up to its maximum size. After the
unmapped pages follow the guard pages which are also guaranteed to generate a
trap when accessed.
unmapped pages follow the offset-guard pages which are also guaranteed to
generate a trap when accessed.
.. inst:: H = static Base, min MinBytes, bound BoundBytes, guard GuardBytes
.. inst:: H = static Base, min MinBytes, bound BoundBytes, offset_guard OffsetGuardBytes
Declare a static heap in the preamble.
@@ -694,17 +694,18 @@ trap when accessed.
:arg MinBytes: Guaranteed minimum heap size in bytes. Accesses below this
size will never trap.
:arg BoundBytes: Fixed heap bound in bytes. This defines the amount of
address space reserved for the heap, not including the guard pages.
:arg GuardBytes: Size of the guard pages in bytes.
address space reserved for the heap, not including the offset-guard
pages.
:arg OffsetGuardBytes: Size of the offset-guard pages in bytes.
Dynamic heaps
~~~~~~~~~~~~~
A *dynamic heap* can be relocated to a different base address when it is
resized, and its bound can move dynamically. The guard pages move when the heap
is resized. The bound of a dynamic heap is stored in a global value.
resized, and its bound can move dynamically. The offset-guard pages move when
the heap is resized. The bound of a dynamic heap is stored in a global value.
.. inst:: H = dynamic Base, min MinBytes, bound BoundGV, guard GuardBytes
.. inst:: H = dynamic Base, min MinBytes, bound BoundGV, offset_guard OffsetGuardBytes
Declare a dynamic heap in the preamble.
@@ -712,14 +713,14 @@ is resized. The bound of a dynamic heap is stored in a global value.
:arg MinBytes: Guaranteed minimum heap size in bytes. Accesses below this
size will never trap.
:arg BoundGV: Global value containing the current heap bound in bytes.
:arg GuardBytes: Size of the guard pages in bytes.
:arg OffsetGuardBytes: Size of the offset-guard pages in bytes.
Heap examples
~~~~~~~~~~~~~
The SpiderMonkey VM prefers to use fixed heaps with a 4 GB bound and 2 GB of
guard pages when running WebAssembly code on 64-bit CPUs. The combination of a
4 GB fixed bound and 1-byte bounds checks means that no code needs to be
offset-guard pages when running WebAssembly code on 64-bit CPUs. The combination
of a 4 GB fixed bound and 1-byte bounds checks means that no code needs to be
generated for bounds checks at all:
.. literalinclude:: heapex-sm64.clif
@@ -728,7 +729,7 @@ generated for bounds checks at all:
A static heap can also be used for 32-bit code when the WebAssembly module
declares a small upper bound on its memory. A 1 MB static bound with a single 4
KB guard page still has opportunities for sharing bounds checking code:
KB offset-guard page still has opportunities for sharing bounds checking code:
.. literalinclude:: heapex-sm32.clif
:language: clif
@@ -738,8 +739,8 @@ If the upper bound on the heap size is too large, a dynamic heap is required
instead.
Finally, a runtime environment that simply allocates a heap with
:c:func:`malloc()` may not have any guard pages at all. In that case, full
bounds checking is required for each access:
:c:func:`malloc()` may not have any offset-guard pages at all. In that case,
full bounds checking is required for each access:
.. literalinclude:: heapex-dyn.clif
:language: clif