#2130: Mulitple comparisons on Word types produce redundant comparisons
-----------------------------------------+----------------------------------
Reporter: dons | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: performance, Word | Testcase:
Architecture: Unknown | Os: Multiple
-----------------------------------------+----------------------------------
Comment (by dons):
The extra comparisons bubble through to the final assembly, as you see
here:
{{{
good :: Int -> Bool
good c | c < 22 = True
| c < 42 = True
| otherwise = False
}}}
The good Int yields:
{{{
good_info:
cmpq $22,7(%rbx)
jl .LcgH
movl $base_GHCziBase_Bool_closure_tbl,%eax
cmpq $42,7(%rbx)
setl %cl
movzbl %cl,%ecx
movq (%rax,%rcx,8),%rbx
addq $8,%rbp
jmp *(%rbp)
.LcgH:
movl $base_GHCziBase_True_closure+2,%ebx
addq $8,%rbp
jmp *(%rbp)
}}}
While our funky Ords for Word:
{{{
bad :: Word -> Bool
bad c | c < 22 = True
| c < 42 = True
| otherwise = False
}}}
yields:
{{{
M_zdwbad_info:
cmpq $22,%rsi
je .Lcm7
cmpq $22,%rsi
jb .Lcmb
cmpq $42,%rsi
je .Lcmd
movl $base_GHCziBase_Bool_closure_tbl,%eax
cmpq $42,%rsi
setb %cl
movzbl %cl,%ecx
movq (%rax,%rcx,8),%rbx
jmp *(%rbp)
.Lcm7:
cmpq $42,%rsi
je .Lcm9
movl $base_GHCziBase_Bool_closure_tbl,%eax
cmpq $42,%rsi
setb %cl
movzbl %cl,%ecx
movq (%rax,%rcx,8),%rbx
jmp *(%rbp)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2130#comment:1>
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