Understanding GHC assembly output

Question

When compiling a haskell source file using the -S option in GHC the assembly code generated is not clear. There\'s no clear distinction between which parts of the assembly c

Answer 1

For top level declarations, it's not too hard. Local definitions can be harder to recognize as their names get mangled and they are likely to get inlined.

Let's see what happens when we compile this simple module.

module Example where

add :: Int -> Int -> Int
add x y = x + y

.data
    .align 8
.globl Example_add_closure
.type Example_add_closure, @object
Example_add_closure:
    .quad   Example_add_info
.text
    .align 8
    .quad   8589934604
    .quad   0
    .quad   15
.globl Example_add_info
.type Example_add_info, @object
Example_add_info:
.LckX:
    jmp base_GHCziBase_plusInt_info
.data
    .align 8
_module_registered:
    .quad   0
.text
    .align 8
.globl __stginit_Example_
.type __stginit_Example_, @object
__stginit_Example_:
.Lcl7:
    cmpq $0,_module_registered
    jne .Lcl8
.Lcl9:
    movq $1,_module_registered
    addq $-8,%rbp
    movq $__stginit_base_Prelude_,(%rbp)
.Lcl8:
    addq $8,%rbp
    jmp *-8(%rbp)
.text
    .align 8
.globl __stginit_Example
.type __stginit_Example, @object
__stginit_Example:
.Lcld:
    jmp __stginit_Example_
.section .note.GNU-stack,"",@progbits
.ident "GHC 7.0.2"

You can see that our function Example.add resulted in the generation of Example_add_closure and Example_add_info. The _closure part, as the name suggests, has to do with closures. The _info part contains the actual instructions of the function. In this case, this is simply a jump to the built-in function GHC.Base.plusInt.

Note that assembly generated from Haskell code looks quite different from what you might get from other languages. The calling conventions are different, and things can get reordered a lot.

In most cases you don't want to jump straight to assembly. It is usually much easier to understand core, a simplified version of Haskell. (Simpler to compile, not necessarily to read). To get at the core, compile with the -ddump-simpl option.

Example.add :: GHC.Types.Int -> GHC.Types.Int -> GHC.Types.Int
[GblId, Arity=2]
Example.add =
  \ (x_abt :: GHC.Types.Int) (y_abu :: GHC.Types.Int) ->
    GHC.Num.+ @ GHC.Types.Int GHC.Num.$fNumInt x_abt y_abu

For some good resources on how to read core, see this question.