mark more things as const in C codegen

Recently i've tried to build ghc-7.8.2 on ia64 and it overflows so called "short data" section. (it's where global 'static' variables live).

I've looked at low-hanging fruits of squashing such global "variables". One source of them seems to be a string literal.

Consider one-line module:

    module B (v) where v = "hello"

in -fvia-C (or unreg) mode it generates code like

    static char gibberish_str[] = "hello";

It uselessly eats data section. The patch switches generator to emit:

    static const char gibberish_str[] = "hello";

Some notes:

1. as far as I see native codegens already put ot to .rodata,

thus it should be safe.

1. I likely didn't cover more cases, like

• RelocatableReadOnlyData

• and mysterious pprTop (CmmData _section (Statics lbl [CmmUninitialised size]))

Size of unreg-stage2 on amd64 before and after the patch:

$ size inplace/lib/bin/ghc-stage2 (unpatched)
   text    data     bss     dec     hex filename
81986382        20776344          44096 102806822       620b526 inplace/lib/bin/ghc-stage2
$ size inplace/lib/bin/ghc-stage2 (patched)
   text    data     bss     dec     hex filename
83648494        19062936          44096 102755526       61fecc6 inplace/lib/bin/ghc-stage2

Text section increased for 1.6MBs (consts moved here, likely .rodata actually), data section decreased for 1.7MBs.

I think with minor fiddling with linker flags we can merge equal constants later on.

Thanks!

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture