Messages in this thread | | | Date | Thu, 14 Sep 2017 09:48:20 -0500 | From | Josh Poimboeuf <> | Subject | Re: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent |
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On Sat, Sep 02, 2017 at 12:32:21PM +0200, Ingo Molnar wrote: > > * Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > > On Thu, Aug 31, 2017 at 12:25:42PM -0500, Josh Poimboeuf wrote: > > > 2) Put "sp" in the clobbers list instead of as an i/o constraint. This > > > mostly works for GCC, and doesn't break clang. However, it causes > > > GCC to insert a "lea -0x10(%rbp),%rsp" in the epilogue of every > > > affected function. > > > > And maybe this extra instruction is negligible for performance and not a > > big deal? I might look at this one after the holiday too. > > Please do statistics of how many functions are affected, on a defconfig-ish > kernel.
As it turns out, the real problem with this option is that it imposes a penalty for CONFIG_FRAME_POINTER=n: even with frame pointers disabled, it forces the frame pointer to be saved for each function which uses the inline asm "call" statements. Our current solution doesn't do that.
- On a defconfig-based kernel, this adds +6k of .text (+0.06%).
- On a Fedora distro-based config, it adds +27k of .text (+0.3%). (I think the difference from defconfig is mostly caused by CONFIG_PARAVIRT.)
I'll try a few more experiments, but I'll probably end up engaging the compiler people as Linus suggested.
-- Josh
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