[GHC] #8311: suboptimal code generated for even :: Int -> Bool by NCG (x86, x86_64)

#8311: suboptimal code generated for even :: Int -> Bool by NCG (x86, x86_64) ------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This appears to be the x86 assembly for a specialization for `even :: Int -> Bool` that's produced by a SPECIALISE pragma for `(^) :: Int -> Int -> Int` in GHC.Real. The relevant bit is offsets `0x2c` through `0x3b`. {{{ 0000000c : c: 8b 45 00 mov 0x0(%ebp),%eax f: c7 45 00 2c 00 00 00 movl $0x2c,0x0(%ebp) 12: R_386_32 .text 16: 89 c6 mov %eax,%esi 18: f7 c6 03 00 00 00 test $0x3,%esi 1e: 75 0c jne 2c 20: ff 26 jmp *(%esi) 22: 66 90 xchg %ax,%ax 24: 00 00 add %al,(%eax) 26: 00 00 add %al,(%eax) 28: 20 00 and %al,(%eax) ... 0000002c : 2c: b8 02 00 00 00 mov $0x2,%eax 31: 89 c1 mov %eax,%ecx 33: 8b 46 03 mov 0x3(%esi),%eax 36: 99 cltd 37: f7 f9 idiv %ecx 39: 85 d2 test %edx,%edx 3b: 75 0b jne 48 3d: be 02 00 00 00 mov $0x2,%esi 3e: R_386_32 ghczmprim_GHCziTypes_True_closure 42: 83 c5 04 add $0x4,%ebp 45: ff 65 00 jmp *0x0(%ebp) 48: be 01 00 00 00 mov $0x1,%esi 49: R_386_32 ghczmprim_GHCziTypes_False_closure 4d: 83 c5 04 add $0x4,%ebp 50: ff 65 00 jmp *0x0(%ebp) }}} It would be much better to do a simple `testb $0x1,0x3(%esi)` in place of the first six instructions of `c8gO_info`. The NCG generates similar code on x86_64. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8311 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler