[Bug tree-optimization/85585] switch to select a string based on an enum can profitably optimize away the table of pointers/offsets into fixed-length char[] blocks. Or use byte offsets into a string
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85585
--- Comment #2 from joseph at codesourcery dot com ---
On Tue, 1 May 2018, peter at cordes dot ca wrote:
> The current strings + pointer-table implementation doesn't merge string
> literals where one string is a suffix of another; this is another a
The linker does that, as well as merging identical strings in different
translation units, via strings going in specially marked string sections
(".section .rodata.str1.1,"aMS",@progbits,1"). These proposed
optimizations would interfere with such linker optimizations, so they
might not be a code size win.
[Bug tree-optimization/85585] switch to select a string based on an enum can profitably optimize away the table of pointers/offsets into fixed-length char[] blocks. Or use byte offsets into a string
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85585 --- Comment #1 from Peter Cordes --- By comparison, the no-PIE table of pointers only needs one instruction: movqCSWTCH.4(,%rdi,8), %rax So all my suggestions cost 1 extra instruction on x86 in no-PIE mode, but at a massive savings in data size. clang -fPIE compiles the plain switch to the obvious / sane 2 instruction sequence which should be our baseline for normal cases. # clang6.0 -fPIE -O3 (switch compilers on the Godbolt link) leaq.Lswitch.table.phy_modes(%rip), %rcx movq(%rcx,%rax,8), %rax Clang is willing to make a table that needs relocations for the entries. (My suggestions all avoid that because they're based on offsets, not a table of pointers. Avoiding rodata relocations that dirty a page and prevent sharing has some non-zero value, although it's low on many architectures where memory is cheap.)
