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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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