On Tue, Sep 29, 2015 at 5:23 PM, Mike Stump <mikest...@comcast.net> wrote:
> On Sep 29, 2015, at 3:40 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> How about adding a "no_caller_saved_registers" attribute?
>
> You can save all call clobbered registers with 3 instructions? Really? I’m
> skeptical. Anyway, if you do this by turning off great swaths of registers,
> then, I guess that doesn’t surprise me. I try and not turn any off in mine.
We need one instruction to save one register.
> Now, if you turn off great swaths of registers, then you can’t actually make
> any calls to an abi that supports those registers as call clobbered:
We don't need to save and restore any registers when calling a
function marked with
"no_caller_saved_registers" attribute except for registers used to
pass parameters.
Maybe "no_caller_saved_registers" isn't a good name.
>> + if (cfun->machine->is_interrupt)
>> + return ((df_regs_ever_live_p (regno)
>> + || (call_used_regs[regno] && cfun->machine->make_calls))
>> + && !fixed_regs[regno]
>> + && !STACK_REGNO_P (regno)
>> + && !MMX_REGNO_P (regno)
>> + && regno != BP_REG
>> + && regno != SP_REG
>> + && (regno <= ST7_REG || regno >= XMM0_REG));
>
> So, any calls to a function that uses any excluded register won’t work, if
> that register is not fixed and is call clobbered. If you call such a
> function, you _must_ save and restore all those registers. Code like:
We save all changed registers as well as all caller-saved registers if we make
any function call. Shouldn't it be sufficient?
>> + if (cfun->machine->is_interrupt && VALID_MMX_REG_MODE (mode))
>> + {
>> + error ("MMX/3Dnow instructions aren't allowed in %s service routine",
>> + (cfun->machine->is_exception ? "exception" : "interrupt"));
>> + return;
>> + }
>
> Does not save your from the obligation of saving such registers, if you
> support function calls inside an interrupt routine. Once you add that
> support to make function calls work, then, you might as well lift this
> restriction, cause it would already just work?
Due to the way how MMX/x87 registers work, we don't support using
MMX/x87 registers in
interrupt handler. People have to write assembly codes if they have
to use them.
> If you really can save everything in 3 instructions, then there is no point
> to trying to enhance it more on your port. 3 instructions execute so fast as
> to not matter.
>
> Now, if you ask me how I know all this, I had to debug a failure to save 1
> register class in the prologue from large multi core instruction trace, and
> that class was the second most important class for general code gen right
> after the gprs. Turns out that I killed a live variable in that class from a
> packet handler interrupt routine cause it failed to save/restore. After
> that, I tested every register class and fixed all the issues.
We include one testcase for each register class in x86, including MMX
and x87, which
gives an error.
--
H.J.
