id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7308	Better code for top-level indirections	simonpj	simonmar	"GHC generates lots of top-level indirections looking like this:
{{{
foo :: T
foo = bar `cast` (some coercion)
}}}
There are good reasons for this. Moreover `foo` will almost never be evaluated, because GHC will inline any call to `foo` at its call site.

'''But''' GHC generates rather a lot of code for this one (almost-vacuous) bindings:
{{{
	.align 4,0x90
	.long	_Swx_srt-(_Foo_zdfTypeableT_info)+4
	.long	0
	.long	65558
.globl _Foo_zdfTypeableT_info
_Foo_zdfTypeableT_info:
.LcwI:
	movl %esi,%eax
	leal -8(%ebp),%ecx
	cmpl 84(%ebx),%ecx
	jb .LcwP
	addl $8,%edi
	cmpl 92(%ebx),%edi
	ja .LcwR
	movl $_stg_CAF_BLACKHOLE_info,-4(%edi)
	movl 100(%ebx),%ecx
	movl %ecx,0(%edi)
	leal -4(%edi),%ecx
	pushl %ecx
	pushl %eax
	pushl %ebx
	movl %eax,76(%esp)
	call _newCAF
	addl $12,%esp
	testl %eax,%eax
	je .LcwK
	movl $_stg_bh_upd_frame_info,-8(%ebp)
	leal -4(%edi),%eax
	movl %eax,-4(%ebp)
	movl $_Foo_zdfTypeableTzuzdctypeOf_closure+1,%esi
	addl $-8,%ebp
	jmp *(%ebp)
.LcwK:
	movl 64(%esp),%eax
	jmp *(%eax)
.LcwR:
	movl $8,116(%ebx)
.LcwP:
	movl %eax,%esi
	jmp *-12(%ebx)
}}}
Whereas all we really need for `foo` is code that says ""jump to `bar`"".  No need to update that top-level thunk. It's just an indirection and (if you have `-O` on) one that will never even be used. 

In short, I think that top-level indirections could do with a tiny special case in the code generator."	bug	closed	high	7.8.1	Compiler	7.6.1	fixed			Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown