Richard Henderson <r...@twiddle.net> writes: > On 06/27/2017 01:39 AM, Alex Bennée wrote: >>> + /* If true, the temp is saved across both basic blocks and >>> + translation blocks. */ >>> + unsigned int temp_global:1; >>> + /* If true, the temp is saved across basic blocks but dead >>> + at the end of translation blocks. If false, the temp is >>> + dead at the end of basic blocks. */ >>> + unsigned int temp_local:1; >>> + unsigned int temp_allocated:1; >> >> This is where my knowledge of the TCG internals gets slightly confused. >> As far as I'm aware all our TranslationBlocks are Basic Blocks - they >> don't have any branches until the end of the block. What is the >> distinction here? >> >> Is a temp_global truly global? I thought the guest state was fully >> rectified by the time we leave the basic block. > > TranslationBlocks are not basic blocks. They normally stop at > branches in the target instruction stream, but they certainly may have > many branches in the tcg opcode stream (brcond and the like). > Consider, for instance, our implementation of arm32's conditional > instructions.
Right. Re-reading the tcg/README it does make the distinction but the term "basic block" is overloaded depending on when talking about the guest instructions or the generated code. > > Beyond that, I agree the language is confusing. > > A temp_global is created by tcg_global_mem_new_*, generally represents > a cpu register, and is synced back to a slot in ENV. > > A temp_local is created by tcg_temp_local_new_*, and is synced to a > slot in the local stack frame. The language from the README: A TCG "temporary" is a variable only live in a basic block. Temporaries are allocated explicitly in each function. A TCG "local temporary" is a variable only live in a function. Local temporaries are allocated explicitly in each function. I must admit I hadn't quite understood the distinction. In the ARM code the only place where tcg_temp_local_new() over tcg_temp_new() is used is in the ld/strex code. I guess because you need to preserve its value over a potential TCG branch? I guess in translate-a64/gen_store_exclusive() the key lines are: TCGv_i64 addr = tcg_temp_local_new_i64(); /* Copy input into a local temp so it is not trashed when the * basic block ends at the branch insn. */ tcg_gen_mov_i64(addr, inaddr); tcg_gen_brcond_i64(TCG_COND_NE, addr, cpu_exclusive_addr, fail_label); > Something without either is simply declared dead at the end of a basic > block, and is a source of confusion to those writing new front-ends. I'll see if I can come up with some improved wording to help new developers in the future. > Anyway, we already have all of these concepts. The change is that > before the patch the only way to tell a temp_global is to compare the > index against tcg_ctx.nb_global. Indeed. As I have previously really only been a consumer of the TCG API I'm taking the opportunity to learn more about the internals as the vector work is likely to touch it ;-) > > > r~ -- Alex Bennée
