On 3/23/24 4:33 AM, Ajit Agarwal wrote: >>> - else if (align_words < GP_ARG_NUM_REG) >>> + else if (align_words < GP_ARG_NUM_REG >>> + || (cum->hidden_string_length >>> + && cum->actual_parm_length <= GP_ARG_NUM_REG)) >> { >> if (TARGET_32BIT && TARGET_POWERPC64) >> return rs6000_mixed_function_arg (mode, type, align_words); >> >> return gen_rtx_REG (mode, GP_ARG_MIN_REG + align_words); >> } >> else >> return NULL_RTX; >> >> The old code for the unused hidden parameter (which was the 9th param) would >> fall thru to the "return NULL_RTX;" which would make the callee assume there >> was a parameter save area allocated. Now instead, we'll return a reg rtx, >> probably of r11 (r3 thru r10 are our param regs) and I'm guessing we'll now >> see a copy of r11 into a pseudo like we do for the other param regs. >> Is that a problem? Given it's an unused parameter, it'll probably get deleted >> as dead code, but could it cause any issues? What if we have more than one >> unused hidden parameter and we return r12 and r13 which have specific uses >> in our ABIs (eg, r13 is our TCB pointer), so it may not actually look dead. >> Have you verified what the callee RTL looks like after expand for these >> unused hidden parameters? Is there a rtx we can return that isn't a NULL_RTX >> which triggers the assumption of a parameter save area, but isn't a reg rtx >> which might lead to some rtl being generated? Would a (const_int 0) or >> something else work? >> >> > For the above use case it will return > > (reg:DI 5 %r5) and below check entry_parm = > (reg:DI 5 %r5) and the following check will not return TRUE and hence > parameter save area will not be allocated.
Why r5?!?! The 8th (integer) param would return r10, so I'd assume if the next param was a hidden param, then it'd get the next gpr, so r11. How does it jump back to r5 which may have been used by the 3rd param? > It will not generate any rtx in the callee rtl code but it just used to > check whether to allocate parameter save area or not when number of args > 8. > > /* If there is no incoming register, we need a stack. */ > entry_parm = rs6000_function_arg (args_so_far, arg); > if (entry_parm == NULL) > return true; > > /* Likewise if we need to pass both in registers and on the stack. */ > if (GET_CODE (entry_parm) == PARALLEL > && XEXP (XVECEXP (entry_parm, 0, 0), 0) == NULL_RTX) > return true; Yes, this code in rs6000_parm_needs_stack() uses the rs6000_function_arg() return value as a boolean to tell us whether a parameter save area is required so what we return is unimportant other than to know it's not NULL_RTX. I'm more concerned about the use of the target hook targetm.calls.function_arg used in the generic parts of the compiler. What will that code do differently now that we return a reg rtx rather than NULL_RTX? Might that code use the reg rtx to emit something? I'd feel better if you could verify what happens in that code when we return a reg rtx for that 9th hidden param which isn't really being passed in a register. Peter