On Thu, Dec 18, 2014 at 2:11 PM, H.J. Lu <hongjiu...@intel.com> wrote: > The Linux kernel never passes floating point arguments around, vararg > functions or not. Hence no vector registers are ever used when calling a > vararg function. But gcc still dutifully emits an "xor %eax,%eax" before > each and every call of a vararg function. Since no callee use that for > anything, these instructions are redundant. > > This patch adds the -mskip-rax-setup option to skip setting up RAX > register when SSE is disabled and there are no variable arguments passed > in vector registers. Since RAX register is used to avoid unnecessarily > saving vector registers on stack when passing variable arguments, the > impacts of this option are callees may waste some stack space, misbehave > or jump to a random location. GCC 4.4 or newer don't those issues, > regardless the RAX register value since they don't check the RAX register > value when SSE is disabled, regardless the RAX register value: > > https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html > > I used it on kernel 3.17.7: > > text data bss dec hex filename > 11493571 2271232 5926912 19691715 12c78c3 vmlinux.skip-rax > 11517879 2271232 5926912 19716023 12cd7b7 vmlinux.orig > > It removed 14309 redundant "xor %eax,%eax" instructions and saved about > 27KB. I am currently running the new kernel without any problem. OK > for trunk?
How about skipping RAX setup unconditionally for !TARGET_SSE? Please see ix86_conditional_register_usage, where SSE registers are squashed for !TARGET_SSE, so it is not possible to use them even in the inline asm. Uros.
