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authorBjörn Gustavsson <[email protected]>2019-06-26 15:49:48 +0200
committerBjörn Gustavsson <[email protected]>2019-08-22 13:37:41 +0200
commit25fe3fb23c594d735cb6ebae120910e44f0cdae4 (patch)
tree746907d20fb667858ec419cf49138f851317e055 /erts/emulator/beam/erl_bif_info.c
parente51727f129d6b5667f81e86865d17cf564712054 (diff)
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Optimize continuation pointer management
The BEAM instructions for calling a function don't save the continuation pointer (return address) on the stack, but to a special BEAM register called CP. It is the responsibility of the called function to save CP to the stack frame before calling other functions. In the earlier implementations of BEAM on Sparc, CP was located in a CPU register. That meant that the continuation pointer was never written to memory when calling simple functions that didn't call other functions at all or ended in a tail-call to another function. The modern BEAM no longer keeps CP in CPU register. Instead, it is kept in the `process` struct (in `p->cp`). That means the continuation pointer must be written to the memory on every call, and if the called function will call other functions, it will must read the continuation pointer from `p->cp` and store it on the stack. This commit eliminates the concept of the CP register and modifies the call instructions to directly store the continuation pointer on the stack. That makes allocation and trimming of stack frames slightly faster. A more important benefit is simplification of code that handles continuation pointers. Because all continuation pointers are now stored on the stack, the special case of handling `p->cp` disappears. Co-authored-by: John Högberg <[email protected]>
Diffstat (limited to 'erts/emulator/beam/erl_bif_info.c')
-rw-r--r--erts/emulator/beam/erl_bif_info.c12
1 files changed, 5 insertions, 7 deletions
diff --git a/erts/emulator/beam/erl_bif_info.c b/erts/emulator/beam/erl_bif_info.c
index aa55bdab6a..dd616645dc 100644
--- a/erts/emulator/beam/erl_bif_info.c
+++ b/erts/emulator/beam/erl_bif_info.c
@@ -2008,14 +2008,16 @@ current_function(Process *c_p, ErtsHeapFactory *hfact, Process* rp,
if (c_p == rp && !(flags & ERTS_PI_FLAG_REQUEST_FOR_OTHER)) {
FunctionInfo fi2;
+ BeamInstr* continuation_ptr;
/*
* The current function is erlang:process_info/{1,2},
* which is not the answer that the application want.
- * We will use the function pointed into by rp->cp
- * instead if it can be looked up.
+ * We will use the continuation pointer stored at the
+ * top of the stack instead.
*/
- erts_lookup_function_info(&fi2, rp->cp, full_info);
+ continuation_ptr = (BeamInstr *) rp->stop[0];
+ erts_lookup_function_info(&fi2, continuation_ptr, full_info);
if (fi2.mfa) {
fi = fi2;
rp->current = fi2.mfa;
@@ -2062,10 +2064,6 @@ current_stacktrace(ErtsHeapFactory *hfact, Process* rp,
s->trace[s->depth++] = rp->i;
depth--;
}
- if (depth > 0 && rp->cp != 0) {
- s->trace[s->depth++] = rp->cp - 1;
- depth--;
- }
erts_save_stacktrace(rp, s, depth);
depth = s->depth;