LLVM and dynamic linking
Aaron Friel
aaron at frieltek.com
Wed Jan 1 22:38:22 UTC 2014
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
1.
LLVM-built GHC, dynamic libs - doesn’t build.
2.
LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable.
3.
GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself.
4.
GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
From: Carter Schonwald<mailto:carter.schonwald at gmail.com>
Sent: Monday, December 30, 2013 1:05 PM
To: Aaron Friel<mailto:aaron at frieltek.com>
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron at frieltek.com<mailto:aaron at frieltek.com>> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
From: Carter Schonwald<mailto:carter.schonwald at gmail.com>
Sent: Friday, December 27, 2013 2:41 PM
To: Ben Gamari<mailto:bgamari.foss at gmail.com>
Cc: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
great work! :)
On Fri, Dec 27, 2013 at 3:21 PM, Ben Gamari <bgamari.foss at gmail.com<mailto:bgamari.foss at gmail.com>> wrote:
Simon Marlow <marlowsd at gmail.com<mailto:marlowsd at gmail.com>> writes:
> This sounds right to me. Did you submit a patch?
>
> Note that dynamic linking with LLVM is likely to produce significantly
> worse code that with the NCG right now, because the LLVM back end uses
> dynamic references even for symbols in the same package, whereas the NCG
> back-end uses direct static references for these.
>
Today with the help of Edward Yang I examined the code produced by the
LLVM backend in light of this statement. I was surprised to find that
LLVM's code appears to be no worse than the NCG with respect to
intra-package references.
My test case can be found here[2] and can be built with the included
`build.sh` script. The test consists of two modules build into a shared
library. One module, `LibTest`, exports a few simple members while the
other module (`LibTest2`) defines members that consume them. Care is
taken to ensure the members are not inlined.
The tests were done on x86_64 running LLVM 3.4 and GHC HEAD with the
patches[1] I referred to in my last message. Please let me know if I've
missed something.
# Evaluation
## First example ##
The first member is a simple `String` (defined in `LibTest`),
helloWorld :: String
helloWorld = "Hello World!"
The use-site is quite straightforward,
testHelloWorld :: IO String
testHelloWorld = return helloWorld
With `-O1` the code looks reasonable in both cases. Most importantly,
both backends use IP relative addressing to find the symbol.
### LLVM ###
0000000000000ef8 <rKw_info>:
ef8: 48 8b 45 00 mov 0x0(%rbp),%rax
efc: 48 8d 1d cd 11 20 00 lea 0x2011cd(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure>
f03: ff e0 jmpq *%rax
0000000000000f28 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>:
f28: eb ce jmp ef8 <rKw_info>
f2a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
### NCG ###
0000000000000d58 <rH1_info>:
d58: 48 8d 1d 71 13 20 00 lea 0x201371(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure>
d5f: ff 65 00 jmpq *0x0(%rbp)
0000000000000d88 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>:
d88: eb ce jmp d58 <rH1_info>
With `-O0` the code is substantially longer but the relocation behavior
is still correct, as one would expect.
Looking at the definition of `helloWorld`[3] itself it becomes clear that
the LLVM backend is more likely to use PLT relocations over GOT. In
general, `stg_*` primitives are called through the PLT. As far as I can
tell, both of these call mechanisms will incur two memory
accesses. However, in the case of the PLT the call will consist of two
JMPs whereas the GOT will consist of only one. Is this a cause for
concern? Could these two jumps interfere with prediction?
In general the LLVM backend produces a few more instructions than the
NCG although this doesn't appear to be related to handling of
relocations. For instance, the inexplicable (to me) `mov` at the
beginning of LLVM's `rKw_info`.
## Second example ##
The second example demonstrates an actual call,
-- Definition (in LibTest)
infoRef :: Int -> Int
infoRef n = n + 1
-- Call site
testInfoRef :: IO Int
testInfoRef = return (infoRef 2)
With `-O1` this produces the following code,
### LLVM ###
0000000000000fb0 <rLy_info>:
fb0: 48 8b 45 00 mov 0x0(%rbp),%rax
fb4: 48 8d 1d a5 10 20 00 lea 0x2010a5(%rip),%rbx # 202060 <rLx_closure>
fbb: ff e0 jmpq *%rax
0000000000000fe0 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>:
fe0: eb ce jmp fb0 <rLy_info>
### NCG ###
0000000000000e10 <rI3_info>:
e10: 48 8d 1d 51 12 20 00 lea 0x201251(%rip),%rbx # 202068 <rI2_closure>
e17: ff 65 00 jmpq *0x0(%rbp)
0000000000000e40 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>:
e40: eb ce jmp e10 <rI3_info>
Again, it seems that LLVM is a bit more verbose but seems to handle
intra-package calls efficiently.
[1] https://github.com/bgamari/ghc/commits/llvm-dynamic
[2] https://github.com/bgamari/ghc-linking-tests/tree/master/ghc-test
[3] `helloWorld` definitions:
LLVM:
00000000000010a8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>:
10a8: 50 push %rax
10a9: 4c 8d 75 f0 lea -0x10(%rbp),%r14
10ad: 4d 39 fe cmp %r15,%r14
10b0: 73 07 jae 10b9 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x11>
10b2: 49 8b 45 f0 mov -0x10(%r13),%rax
10b6: 5a pop %rdx
10b7: ff e0 jmpq *%rax
10b9: 4c 89 ef mov %r13,%rdi
10bc: 48 89 de mov %rbx,%rsi
10bf: e8 0c fd ff ff callq dd0 <newCAF at plt>
10c4: 48 85 c0 test %rax,%rax
10c7: 74 22 je 10eb <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x43>
10c9: 48 8b 0d 18 0f 20 00 mov 0x200f18(%rip),%rcx # 201fe8 <_DYNAMIC+0x228>
10d0: 48 89 4d f0 mov %rcx,-0x10(%rbp)
10d4: 48 89 45 f8 mov %rax,-0x8(%rbp)
10d8: 48 8d 05 21 00 00 00 lea 0x21(%rip),%rax # 1100 <cJC_str>
10df: 4c 89 f5 mov %r14,%rbp
10e2: 49 89 c6 mov %rax,%r14
10e5: 58 pop %rax
10e6: e9 b5 fc ff ff jmpq da0 <ghczmprim_GHCziCString_unpackCStringzh_info at plt>
10eb: 48 8b 03 mov (%rbx),%rax
10ee: 5a pop %rdx
10ef: ff e0 jmpq *%rax
NCG:
0000000000000ef8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>:
ef8: 48 8d 45 f0 lea -0x10(%rbp),%rax
efc: 4c 39 f8 cmp %r15,%rax
eff: 72 3f jb f40 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x48>
f01: 4c 89 ef mov %r13,%rdi
f04: 48 89 de mov %rbx,%rsi
f07: 48 83 ec 08 sub $0x8,%rsp
f0b: b8 00 00 00 00 mov $0x0,%eax
f10: e8 1b fd ff ff callq c30 <newCAF at plt>
f15: 48 83 c4 08 add $0x8,%rsp
f19: 48 85 c0 test %rax,%rax
f1c: 74 20 je f3e <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x46>
f1e: 48 8b 1d cb 10 20 00 mov 0x2010cb(%rip),%rbx # 201ff0 <_DYNAMIC+0x238>
f25: 48 89 5d f0 mov %rbx,-0x10(%rbp)
f29: 48 89 45 f8 mov %rax,-0x8(%rbp)
f2d: 4c 8d 35 1c 00 00 00 lea 0x1c(%rip),%r14 # f50 <cGG_str>
f34: 48 83 c5 f0 add $0xfffffffffffffff0,%rbp
f38: ff 25 7a 10 20 00 jmpq *0x20107a(%rip) # 201fb8 <_DYNAMIC+0x200>
f3e: ff 23 jmpq *(%rbx)
f40: 41 ff 65 f0 jmpq *-0x10(%r13)
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