On Sat, Jul 9, 2016 at 12:00 PM, Dmitriy - <dima00...@gmail.com> wrote:
> Hello,
>
> I try to understand what happens in this code:
> When you call something you are writing:
> __ Call(builtin, RelocInfo::CODE_TARGET);
>
> What how it looks:
> void MacroAssembler::Call(Handle<Code> code_object,
> RelocInfo::Mode rmode,
> TypeFeedbackId ast_id) {
> #ifdef DEBUG
> int end_position = pc_offset() + CallSize(code_object);
> #endif
> DCHECK(RelocInfo::IsCodeTarget(rmode) ||
> rmode == RelocInfo::CODE_AGE_SEQUENCE);
> call(code_object, rmode, ast_id);
> #ifdef DEBUG
> CHECK_EQ(end_position, pc_offset());
> #endif
> }
>
> Then you go to call(code_object, rmode, ast_id);
>
> void Assembler::call(Handle<Code> target,
> RelocInfo::Mode rmode,
> TypeFeedbackId ast_id) {
> positions_recorder()->WriteRecordedPositions();
> EnsureSpace ensure_space(this);
> // 1110 1000 #32-bit disp.
> emit(0xE8);
> emit_code_target(target, rmode, ast_id);
> }
>
> Finally:
>
> void Assembler::emit_code_target(Handle<Code> target,
> RelocInfo::Mode rmode,
> TypeFeedbackId ast_id) {
> DCHECK(RelocInfo::IsCodeTarget(rmode) ||
> rmode == RelocInfo::CODE_AGE_SEQUENCE);
> if (rmode == RelocInfo::CODE_TARGET && !ast_id.IsNone()) {
> RecordRelocInfo(RelocInfo::CODE_TARGET_WITH_ID, ast_id.ToInt());
> } else {
> RecordRelocInfo(rmode);
> }
> int current = code_targets_.length();
> if (current > 0 && code_targets_.last().is_identical_to(target)) {
> // Optimization if we keep jumping to the same code target.
> emitl(current - 1);
> } else {
> code_targets_.Add(target);
> emitl(current);
> }
> }
>
> I understand all code besides one things:
> int current = code_targets_.length();
> emitl(current);
>
> We write index of code object in code_targets, why we don't write address to
> object directly?
> I grep the code and I've not find appropriate using of code_targets.
> But with --print_code I can see that v8 generates call by address, not a
> index.
>
> Do you know where v8 translates index from code_targets to appropriate
> address?
I think it's Code::CopyFrom() in src/objects.cc, which runs as a
fix-up phase in Factory::NewCode() in src/factory.cc.
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