Better code for top-level indirections

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.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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