With this patch, the LowerGC transformation becomes the
ShadowStackCollector, which additionally has reduced overhead with
no sacrifice in portability.
Considering a function @fun with 8 loop-local roots,
ShadowStackCollector introduces the following overhead
(x86):
; shadowstack prologue
movl L_llvm_gc_root_chain$non_lazy_ptr, %eax
movl (%eax), %ecx
movl $___gc_fun, 20(%esp)
movl $0, 24(%esp)
movl $0, 28(%esp)
movl $0, 32(%esp)
movl $0, 36(%esp)
movl $0, 40(%esp)
movl $0, 44(%esp)
movl $0, 48(%esp)
movl $0, 52(%esp)
movl %ecx, 16(%esp)
leal 16(%esp), %ecx
movl %ecx, (%eax)
; shadowstack loop overhead
(none)
; shadowstack epilogue
movl 48(%esp), %edx
movl %edx, (%ecx)
; shadowstack metadata
.align 3
___gc_fun: # __gc_fun
.long 8
.space 4
In comparison to LowerGC:
; lowergc prologue
movl L_llvm_gc_root_chain$non_lazy_ptr, %eax
movl (%eax), %ecx
movl %ecx, 48(%esp)
movl $8, 52(%esp)
movl $0, 60(%esp)
movl $0, 56(%esp)
movl $0, 68(%esp)
movl $0, 64(%esp)
movl $0, 76(%esp)
movl $0, 72(%esp)
movl $0, 84(%esp)
movl $0, 80(%esp)
movl $0, 92(%esp)
movl $0, 88(%esp)
movl $0, 100(%esp)
movl $0, 96(%esp)
movl $0, 108(%esp)
movl $0, 104(%esp)
movl $0, 116(%esp)
movl $0, 112(%esp)
; lowergc loop overhead
leal 44(%esp), %eax
movl %eax, 56(%esp)
leal 40(%esp), %eax
movl %eax, 64(%esp)
leal 36(%esp), %eax
movl %eax, 72(%esp)
leal 32(%esp), %eax
movl %eax, 80(%esp)
leal 28(%esp), %eax
movl %eax, 88(%esp)
leal 24(%esp), %eax
movl %eax, 96(%esp)
leal 20(%esp), %eax
movl %eax, 104(%esp)
leal 16(%esp), %eax
movl %eax, 112(%esp)
; lowergc epilogue
movl 48(%esp), %edx
movl %edx, (%ecx)
; lowergc metadata
(none)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45670 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/runtime/GC/SemiSpace/semispace.c b/runtime/GC/SemiSpace/semispace.c
index b8ef188..40af1cb 100644
--- a/runtime/GC/SemiSpace/semispace.c
+++ b/runtime/GC/SemiSpace/semispace.c
@@ -97,24 +97,26 @@
* FIXME: This should be in a code-generator specific library, but for now this
* will work for all code generators.
*/
-typedef struct GCRoot {
- void **RootPtr;
- void *Meta;
-} GCRoot;
+struct FrameMap {
+ int32_t NumRoots; // Number of roots in stack frame.
+ int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
+ void *Meta[]; // May be absent for roots without metadata.
+};
-typedef struct GCRoots {
- struct GCRoots *Next;
- unsigned NumRoots;
- GCRoot RootRecords[];
-} GCRoots;
-GCRoots *llvm_gc_root_chain;
+struct StackEntry {
+ ShadowStackEntry *Next; // Caller's stack entry.
+ const FrameMap *Map; // Pointer to constant FrameMap.
+ void *Roots[]; // Stack roots (in-place array).
+};
+StackEntry *llvm_gc_root_chain;
void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta)) {
- GCRoots *R = llvm_gc_root_chain;
- for (; R; R = R->Next) {
+ for (StackEntry *R; R; R = R->Next) {
unsigned i, e;
- for (i = 0, e = R->NumRoots; i != e; ++i)
- FP(R->RootRecords[i].RootPtr, R->RootRecords[i].Meta);
+ for (i = 0, e = R->NumMeta; i != e; ++i)
+ FP(&R->Roots[i], R->Map->Meta[i]);
+ for (e = R->NumRoots; i != e; ++i)
+ FP(&R->Roots[i], NULL);
}
}
/* END FIXME! */