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| author | Eli Friedman <eli.friedman@gmail.com> | 2010-07-03 07:38:12 +0000 |
|---|---|---|
| committer | Eli Friedman <eli.friedman@gmail.com> | 2010-07-03 07:38:12 +0000 |
| commit | b4a74c1d820bd913e91e4c921a141824f6bfe2df (patch) | |
| tree | b22c5c0be1bd40666c4d4f16f01defed19afdd0d /lib/Target/README.txt | |
| parent | 68b559e5f34b787fac04eb617c451cc840a473b4 (diff) | |
| download | llvm-b4a74c1d820bd913e91e4c921a141824f6bfe2df.tar.gz llvm-b4a74c1d820bd913e91e4c921a141824f6bfe2df.tar.bz2 llvm-b4a74c1d820bd913e91e4c921a141824f6bfe2df.tar.xz | |
Note switch-lowering inefficiency.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107565 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Target/README.txt')
| -rw-r--r-- | lib/Target/README.txt | 36 |
1 files changed, 36 insertions, 0 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index b1080b594f..8f785393fd 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -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. + +//===---------------------------------------------------------------------===// |