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Note switch-lowering inefficiency.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107565 91177308-0d34-0410-b5e6-96231b3b80d8
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Eli Friedman 2010-07-03 07:38:12 +00:00
parent 68b559e5f3
commit b4a74c1d82
1 changed files with 36 additions and 0 deletions
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36
lib/Target/README.txt
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@ -1888,3 +1888,39 @@ of the ADD, and replacing the ADD with the ADDE, should give the desired result.
(That said, we are doing a lot better than gcc on this testcase. :) )
//===---------------------------------------------------------------------===//
Switch lowering generates less than ideal code for the following switch:
define void @a(i32 %x) nounwind {
entry:
switch i32 %x, label %if.end [
i32 0, label %if.then
i32 1, label %if.then
i32 2, label %if.then
i32 3, label %if.then
i32 5, label %if.then
]
if.then:
tail call void @foo() nounwind
ret void
if.end:
ret void
}
declare void @foo()
Generated code on x86-64 (other platforms give similar results):
a:
cmpl $5, %edi
ja .LBB0_2
movl %edi, %eax
movl $47, %ecx
btq %rax, %rcx
jb .LBB0_3
.LBB0_2:
ret
.LBB0_3:
xorb %al, %al
jmp foo@PLT # TAILCALL
The movl+movl+btq+jb could be simplified to a cmpl+jne.
//===---------------------------------------------------------------------===//