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:[email protected]> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:[email protected]> 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 <[email protected]<mailto:[email protected]>> 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:[email protected]> Sent: Friday, December 27, 2013 2:41 PM To: Ben Gamari<mailto:[email protected]> Cc: [email protected]<mailto:[email protected]> great work! :) On Fri, Dec 27, 2013 at 3:21 PM, Ben Gamari <[email protected]<mailto:[email protected]>> wrote: Simon Marlow <[email protected]<mailto:[email protected]>> 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@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@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@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) _______________________________________________ ghc-devs mailing list [email protected]<mailto:[email protected]> http://www.haskell.org/mailman/listinfo/ghc-devs
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