On Thu, Dec 18, 2014 at 2:24 PM, Uros Bizjak <ubiz...@gmail.com> wrote: > 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.
... when -ffreestanding is in effect, of course. Uros.
