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    SubjectCompiling x86 with and without frame pointer
    Date
    The conventional wisdom is that compiling x86 without frame pointer
    results in smaller code. It turns out to be the opposite, compiling
    with frame pointers results in a smaller kernel. gcc version 3.2
    20020822 (Red Hat Linux Rawhide 3.2-4).

    # size 2.4.20-rc2-*/vmlinux
    text data bss dec hex filename
    2669584 337972 402697 3410253 34094d 2.4.20-rc2-fp/vmlinux
    2676919 337972 402697 3417588 3425f4 2.4.20-rc2-nofp/vmlinux

    Without frame pointers, vmlinux is 7K bigger. The difference is that
    code with frame pointers can use ebp to directly access the stack,
    without frame pointers it has to use esp with an index.

    With frame pointers:

    00000c10 <inet_dgram_connect>:
    c10: 55 push %ebp
    c11: 89 e5 mov %esp,%ebp
    c13: 83 ec 14 sub $0x14,%esp
    c16: 89 75 fc mov %esi,0xfffffffc(%ebp)
    c19: 8b 45 08 mov 0x8(%ebp),%eax
    c1c: 8b 75 0c mov 0xc(%ebp),%esi
    c1f: 89 5d f8 mov %ebx,0xfffffff8(%ebp)
    c22: 8b 58 18 mov 0x18(%eax),%ebx
    c25: 66 83 3e 00 cmpw $0x0,(%esi)
    c29: 74 3d je c68 <inet_dgram_connect+0x58>

    Without frame pointers:

    00000c10 <inet_dgram_connect>:
    c10: 83 ec 14 sub $0x14,%esp
    c13: 8b 44 24 18 mov 0x18(%esp,1),%eax
    c17: 89 74 24 10 mov %esi,0x10(%esp,1)
    c1b: 8b 74 24 1c mov 0x1c(%esp,1),%esi
    c1f: 89 5c 24 0c mov %ebx,0xc(%esp,1)
    c23: 8b 58 18 mov 0x18(%eax),%ebx
    c26: 66 83 3e 00 cmpw $0x0,(%esi)
    c2a: 74 44 je c70 <inet_dgram_connect+0x60>

    The difference is that stack accesses via ebp are 3 bytes, stack
    accesses via esp+index are 4 bytes. On any function with a large
    number of stack accesses, this quickly outweighs the extra prologue
    code for frame pointers.

    The smaller instruction set will improve icache usage. Whether this is
    offset by the increased register pressure is something for
    benchmarking. Any of the benchmarkers care to test x86 kernels with
    and without frame pointers?

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