LLVM and dynamic linking
Simon Marlow
marlowsd at gmail.com
Wed Jan 8 10:19:40 UTC 2014
On 27/12/13 20:21, Ben Gamari wrote:
> Simon Marlow <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.
This is good news, however what worries me is that I still don't
understand *why* you got these results. Where in the LLVM backend is
the magic that does something special for intra-package references? I
know where it is in the NCG backend - CLabel.labelDynamic - but I can't
see this function used at all in the LLVM backend. So what is the
mechanism that lets LLVM optimise these calls? Is it happening
magically in the linker, perhaps? But that would only be possible when
using -Bsymbolic or -Bsymbolic-functions, which is a choice made at link
time.
As far as I can tell, all we do is pass a flag to llc to tell it to
compile for dynamic/PIC, in DriverPipeline.runPhase.
Cheers,
Simon
>
>
> # 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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