On Fri, 2016-07-29 at 06:19 +0530, Richard Henderson wrote: > (1) The helper, since it writes to registers controlled by tcg, must be > described to clobber all registers. Which will noticeably increase memory > traffic to ENV. For instance, you won't be able to hold the guest register > holding the address in a host register across the call.
So after fixing my test setup, I did observe indeed a small performance loss using the helper in qemu-user. It might still win us something in softmmu due to avoiding extra translations but I will leave that aside as I mentioned separately. Now out of curosity, I tried this: --- a/target-ppc/helper.h +++ b/target-ppc/helper.h @@ -22,12 +22,12 @@ DEF_HELPER_1(check_tlb_flush, void, env) #endif DEF_HELPER_3(lmw, void, env, tl, i32) -DEF_HELPER_3(stmw, void, env, tl, i32) +DEF_HELPER_FLAGS_3(stmw, TCG_CALL_NO_WG, void, env, tl, i32) DEF_HELPER_4(lsw, void, env, tl, i32, i32) DEF_HELPER_5(lswx, void, env, tl, i32, i32, i32) -DEF_HELPER_4(stsw, void, env, tl, i32, i32) -DEF_HELPER_3(dcbz, void, env, tl, i32) -DEF_HELPER_2(icbi, void, env, tl) +DEF_HELPER_FLAGS_4(stsw, TCG_CALL_NO_WG, void, env, tl, i32, i32) +DEF_HELPER_FLAGS_3(dcbz, TCG_CALL_NO_WG, void, env, tl, i32) +DEF_HELPER_FLAGS_2(icbi, TCG_CALL_NO_WG, void, env, tl) DEF_HELPER_5(lscbx, tl, env, tl, i32, i32, i32) #if defined(TARGET_PPC64) If my understanding is right, the above is correct, as none of these instructions will write to the env, though they can read from it and/ or generate faults. Sadly I haven't observed any performance improvement as a result in a few micro-benchmarks I cooked up. Cheers, Ben
