[LLVMbugs] [Bug 23258] New: Add -mskip-rax-setup option for x86-64

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23258

            Bug ID: 23258
           Summary: Add -mskip-rax-setup option for x86-64
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: hjl.tools at gmail.com
                CC: llvmbugs at cs.uiuc.edu, michael.m.kuperstein at intel.com
    Classification: Unclassified

GCC 5 added a compiler option, -mskip-rax-setup, for x86-64.  It skips
setting up the RAX register when SSE is disabled and there are no
variable arguments passed in vector registers.  Since kernel doesn't
pass vector registers to functions with variable arguments, this option
can be used to optimize the x86-64 kernel.  For kernel 3.17:

    text   data    bss    dec      hex   filename
11455921 2204048 5853184 19513153 129bf41 vmlinux #with -mskip-rax-setup
11480079 2204048 5853184 19537311 12a1d9f vmlinux

It will be nice to implement it in clang.

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