Assembly decoding help?
I'm trying to get a feel for the assembly output by GHC on my
platform. Below is a module containing one function and the associated
assembly. I've put in comments what I think is going on, but I'd
appreciate it if anyone could give me some pointers. I'd really like
to know three things:
* Why does _Add_unsafeShiftR_info check if (%esi) is 3?
* What's going on in _s86_info?
* At the end of _s87_info, 8 is added to %ebp and then jumped to. Is
that a jump to the I# constructor and, if so, how did it's address get
to that offset from %ebp?
Thanks in advance for any assistance!
Justin
--- cut here
# Compiled with
#
# ghc -c Add.hs -fext-core -keep-s-files -fasm -O2 -ddump-to-file
-ddump-stg -ddump-cmm
#
# Platform: Windows XP.
# Processor Pentium 4.
# GHC Version: 6.8.2.
#
# {-# OPTIONS_GHC -fglasgow-exts -fbang-patterns #-}
# module Add (unsafeShiftR)
#
# where
#
# import Data.Bits
# import GHC.Base (uncheckedShiftRL#, Int(..), word2Int#, int2Word#)
#
# I# a `unsafeShiftR` I# b = I# (word2Int# (int2Word# a
`uncheckedShiftRL#` b))
#
.data
.align 4
.globl _Add_unsafeShiftR_closure
_Add_unsafeShiftR_closure:
.long _Add_unsafeShiftR_info # Specifies entry point for unsafeShiftR?
.text
.align 4,0x90
.long 33
.long 34
_s87_info:
addl $8,%edi
cmpl 92(%ebx),%edi # Stack
check?
ja .Lc8n
movl 4(%ebp),%eax
# Get value to be shifted
movl 3(%esi),%ecx
# Get shift amount
shrl %cl,%eax
# Shift value
movl $_base_GHCziBase_Izh_con_info,-4(%edi) # Get ready to call I#?
movl %eax,(%edi)
# Constructor value
leal -3(%edi),%esi #
Address to what in I#?
addl $8,%ebp
# Make room on stack for next caller?
jmp *(%ebp)
# Go to constructor? Tail call?
.Lc8n:
movl $8,112(%ebx)
jmp *-8(%ebx)
# Error routine if stack overflows?
.text
.align 4,0x90
.long 1
.long 34
_s86_info:
movl 4(%ebp),%eax
# Testing what here?
movl 3(%esi),%ecx
movl %ecx,4(%ebp)
movl %eax,%esi
movl $_s87_info,(%ebp)
testl $3,%esi
jne _s87_info
jmp *(%esi)
.text
.align 4,0x90
.long 131084
.long 0
.long 15
.globl _Add_unsafeShiftR_info
_Add_unsafeShiftR_info:
movl (%ebp),%esi
movl $_s86_info,(%ebp)
testl $3,%esi
# Looking at StgRetInfoTable here?
jne _s86_info
# Enter body of unsafeShiftR?
jmp *(%esi)
.data
.align 4
__module_registered:
.long 0
.text
.align 4,0x90
.globl ___stginit_Add_
___stginit_Add_:
cmpl $0,__module_registered
jne .Lc8E
.Lc8F:
movl $1,__module_registered
addl $-4,%ebp
movl $___stginit_base_GHCziBase_,(%ebp)
addl $-4,%ebp
movl $___stginit_base_Prelude_,(%ebp)
addl $-4,%ebp
movl $___stginit_base_DataziBits_,(%ebp)
.Lc8E:
addl $4,%ebp
jmp *-4(%ebp)
.text
.align 4,0x90
.globl ___stginit_Add
___stginit_Add:
jmp ___stginit_Add_
.ident GHC 6.8.2
___
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Re: Assembly decoding help?
On Tue, Mar 04, 2008 at 05:07:03PM -0800, Justin Bailey wrote: I'm trying to get a feel for the assembly output by GHC on my platform. Below is a module containing one function and the associated assembly. I've put in comments what I think is going on, but I'd appreciate it if anyone could give me some pointers. I'd really like to know three things: * Why does _Add_unsafeShiftR_info check if (%esi) is 3? * What's going on in _s86_info? * At the end of _s87_info, 8 is added to %ebp and then jumped to. Is that a jump to the I# constructor and, if so, how did it's address get to that offset from %ebp? Thanks in advance for any assistance! It would be more helpful if you didn't try to go from Haskell to assembly in one step - it's a lot easier to understand each big step of the GHC pipeline individually. Haskell | \- Core (ghc -ddump-simpl Foo.hs Foo.core; or -fext-core if you want something ugly but parsable; an unrestricted but simple expression-functional language) | \- STG (ghc -ddump-stg ...) (Much more regular than Core; more like functional C) | \- C-- (ghc -ddump-cmm) (just what it says: Simplified C for compiler writers. The universal assembly language for the 21st century) | \- assembly Stefan signature.asc Description: Digital signature ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Assembly decoding help?
On 3/4/08, Justin Bailey [EMAIL PROTECTED] wrote: I'm trying to get a feel for the assembly output by GHC on my platform. Below is a module containing one function and the associated assembly. I've put in comments what I think is going on, but I'd appreciate it if anyone could give me some pointers. I'd really like to know three things: * Why does _Add_unsafeShiftR_info check if (%esi) is 3? * What's going on in _s86_info? * At the end of _s87_info, 8 is added to %ebp and then jumped to. Is that a jump to the I# constructor and, if so, how did it's address get to that offset from %ebp? I agree with Stefan's advice: read the STG code, especially to answer the second two questions (-ddump-stg), and read the STG paper (if you haven't already): Implementing lazy functional languages on stock hardware: the Spineless Tagless G-machine, SL Peyton Jones, Journal of Functional Programming 2(2), Apr 1992, pp127-202. Then if anything's still not clear, ask on the mailing list. Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt The future is not google-able. -- William Gibson ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Assembly decoding help?
Justin Bailey wrote: I'm trying to get a feel for the assembly output by GHC on my platform. Below is a module containing one function and the associated assembly. I've put in comments what I think is going on, but I'd appreciate it if anyone could give me some pointers. I'd really like to know three things: * Why does _Add_unsafeShiftR_info check if (%esi) is 3? * What's going on in _s86_info? * At the end of _s87_info, 8 is added to %ebp and then jumped to. Is that a jump to the I# constructor and, if so, how did it's address get to that offset from %ebp? One point that seems to irritate you is that ghc does pointer tagging; the lower 2 bits of a pointer are zero if it points to an unevaluated closure (a thunk); otherwise they encode a constructor. (I'm not sure what happens in the case when there are more than 3 constructors. But in your case there's only one, I#, with tag 01.) Register usage (as far as I can see): %ebp - STG stack pointer %edi - Heap pointer (in particular cmpl 92(%ebx),%edi is a heap check and will trigger a GC) %esi - at start of *_info, points to most recently evaluated value. %ebx - points to some RTS global variables. Rough outline of the code: _Add_unsafeShiftR_info: find first argument, and evaluate it if it's not yet fully evaluated. continue at _s86_info. (note that a pointer to _s86_info, a continuation, is stored on the control stack) _s86_info: store the first argument value on the stack, load the second one, and evaluate it if necessary. continue at _s87_info. _s87_info: load both arguments, and compute result. The result is fully evaluated, so a tagged pointer is returned. (the -3 is -4, plus 1 for the tag) clean up the stack (the space for the two arguments is no longer needed) and jump to the continuation stored by the caller. HTH, Bertram ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
