#14666: Improve assembly for dense jump tables. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 (CodeGen) | Keywords: Cmm, Asm, | Operating System: Unknown/Multiple CodeGen | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When generating assembly for cases with dense alternatives GHC currently often generates somewhat repetitive code: {{{ Main.$wf_test_info: ... # Check range _u48M: #Jump into jump table jmp *_n48P(,%r14,8) _c48H: movl $11008,%ebx jmp *(%rbp) ##################### # Repeat 6 more times ##################### _c48A: movl $11001,%ebx jmp *(%rbp) _c48z: movq $-1,%rbx jmp *(%rbp) _n48P: #Jump table }}} From what I've seen it should often be possible to replace the indirect jmp with a indirect mov instead. {{{ ... #Check Range .Lu48M: #Get value out of table and jump to continuation. movl .Ln48P(,%r14,8), %ebx jmp *(%rbp) .Lc48z: movq $-1,%rbx jmp *(%rbp) .Ln48P: #Jump table }}} Depending on the number of cases this has the potential to do a lot for code size. ---- I did the transformation manually for one case in a inner loop. It improved speed but not much and depending on the codelayout I did it was possible to get worse performance in specific cases. It's also not a simply patch as Cmm currently generates switches containing gotos. So this would require a change in the Cmm stage as well as in the code generator. But it should be at least worth investigating and seems to be what gcc does for similar cases. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14666 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler