[GHC] #14359: C-- pipeline/NCG fails to optimize simple repeated addition

#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 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: -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler