#14359: C-- pipeline/NCG fails to optimize simple repeated addition -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:

While debugging #14346 I noticed some rather abhorrent code in a disassembly of the `newPinnedByteArray#` primop: {{{ Dump of assembler code for function stg_newPinnedByteArrayzh: 0x00000000004a8518 <+0>: mov 0x378(%r13),%rax 0x00000000004a851f <+7>: cmpq $0x0,0x10(%rax) 0x00000000004a8524 <+12>: je 0x4a8593 0x00000000004a8526 <+14>: mov 0x4f5730,%rax 0x00000000004a852e <+22>: mov 0x38(%rax),%rax 0x00000000004a8532 <+26>: cmp 0x4f5718,%rax 0x00000000004a853a <+34>: jae 0x4a8593 0x00000000004a853c <+36>: mov %rbx,%rax 0x00000000004a853f <+39>: lea 0x7(%rax),%rcx 0x00000000004a8543 <+43>: shr $0x3,%rcx 0x00000000004a8547 <+47>: add $0x10,%rax <--- starts here 0x00000000004a854b <+51>: add $0xf,%rax 0x00000000004a854f <+55>: add $0x7,%rax 0x00000000004a8553 <+59>: shr $0x3,%rax 0x00000000004a8557 <+63>: mov $0x49d820,%ecx }}} That is three successive `add` instructions; surely those should be collapsed into one by the Cmm-to-Cmm pipeline.

New description: While debugging #14346 I noticed some rather abhorrent code in a disassembly of the `newPinnedByteArray#` primop: {{{ Dump of assembler code for function stg_newPinnedByteArrayzh: 0x00000000004a8518 <+0>: mov 0x378(%r13),%rax 0x00000000004a851f <+7>: cmpq $0x0,0x10(%rax) 0x00000000004a8524 <+12>: je 0x4a8593 0x00000000004a8526 <+14>: mov 0x4f5730,%rax 0x00000000004a852e <+22>: mov 0x38(%rax),%rax 0x00000000004a8532 <+26>: cmp 0x4f5718,%rax 0x00000000004a853a <+34>: jae 0x4a8593 0x00000000004a853c <+36>: mov %rbx,%rax 0x00000000004a853f <+39>: lea 0x7(%rax),%rcx 0x00000000004a8543 <+43>: shr $0x3,%rcx 0x00000000004a8547 <+47>: add $0x10,%rax <--- starts here 0x00000000004a854b <+51>: add $0xf,%rax 0x00000000004a854f <+55>: add $0x7,%rax 0x00000000004a8553 <+59>: shr $0x3,%rax 0x00000000004a8557 <+63>: mov $0x49d820,%ecx ... }}} That is three successive `add` instructions; surely those should be collapsed into one by the Cmm-to-Cmm pipeline. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14359#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14359: C-- pipeline/NCG fails to optimize simple repeated addition -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): Well that is a relief. I guess this might just be an artifact from the fact I was using a `validate` build. I'll have to check this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14359#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler