[llvm-commits] PATCH: replace if-chain in AsmParser's directive parsing code
Eli Bendersky
eliben at google.com
Tue Jan 8 09:43:52 PST 2013
On Thu, Jan 3, 2013 at 9:53 AM, Eli Bendersky <eliben at google.com> wrote:
> On Wed, Dec 19, 2012 at 3:04 PM, Eli Bendersky <eliben at google.com> wrote:
>> Ping?
>
> Ping #2
>
Ping #3
Eli
>
>
>>
>> On Tue, Dec 18, 2012 at 2:05 PM, Eli Bendersky <eliben at google.com> wrote:
>>>>> AsmParser has a couple of if-chains when parsing directives. The
>>>>> attached patch replaces the if-chains with enum+switch. No change in
>>>>> functionality.
>>>>>
>>>>> Unfortunately I didn't measure any significant speedups for the whole
>>>>> llvm-mc (only 1-2%), so I'll do more fine-grained measurements once I
>>>>> get the opportunity.
>>>>>
>>>>> Also, I should add that in general AsmParser's parsing code can be
>>>>> cleaned up even more (for example w.r.t. the extensions registersing
>>>>> directive parsers, which can be unified with other directive parsing
>>>>> code). So this patch should just be seen as a first step in a
>>>>> (hopefully) right direction.
>>>>
>>>> Just MHO, but this makes the code more complex and hopefully won't provide a speedup.
>>>> We generally expect that:
>>>>
>>>> StringRef X = …
>>>>
>>>> if (X == "foo")
>>>> return ...
>>>> if (x == "bar")
>>>> return …
>>>>
>>>> to be turned into a switch on X[0].
>>>>
>>>
>>> I may be missing something, but this code:
>>>
>>> int main(int argc, char** argv) {
>>> StringRef r(argv[1]);
>>>
>>> if (r == "something")
>>> return 17;
>>> if (r == "foo")
>>> return 31;
>>> if (r == "bar")
>>> return 71;
>>> if (r == "baz")
>>> return 97;
>>>
>>> return 0;
>>> }
>>>
>>> When compiled with 'clang -O3' (Clang 3.2) does not produce a jump
>>> table, but a chain of tests and jumps as usual from an if-chain (**).
>>> I tried several variations (else-ifs...), and the result is similar.
>>> By the way, this is also what GCC does (and I suspect that many
>>> developers have their LLVM compiled with GCC).
>>> Perhaps I am missing something?
>>>
>>> Besides, the actual pattern in AsmParser is not a pure "if X return Y"
>>> chain. Some "then" statements don't return, and some conditions are
>>> logical ANDs of several tests, and so on. So detecting a clean switch
>>> pattern could be difficult.
>>>
>>>> Just MHO, but this makes the code more complex and hopefully won't provide a speedup.
>>>
>>> To address the "more complex" point. Whimsically, it would be hard to
>>> make that code path in AsmParser more complex ;-) But seriously, a
>>> longer-term plan would be to combine this lookup table with another
>>> that already exists in the same function - DirectiveMap, which is used
>>> for assembler extensions registering themselves on certain directives.
>>> My thinking was to first get the general if-chain removal approved,
>>> and then try to fuse the two lookup tables to attain some level of
>>> consistency in the code. IMHO the end result would be much cleaner and
>>> more readable than what it has now.
>>>
>>> (**) Methodology:
>>>
>>> $ ~/llvm/3.2rc2/bin/clang++ -fno-rtti -O3 stringref_switch.cpp
>>> `~/llvm/3.2rc2/bin/llvm-config --cxxflags --libs`
>>> `~/llvm/3.2rc2/bin/llvm-config --ldflags` -o stringref_switch
>>>
>>> Followed by objdump -d. Here's a typical piece of assembly output:
>>>
>>> 4005c0: 53 push %rbx
>>> 4005c1: 48 8b 5e 08 mov 0x8(%rsi),%rbx
>>> 4005c5: 48 89 df mov %rbx,%rdi
>>> 4005c8: e8 e3 fe ff ff callq 4004b0 <strlen at plt>
>>> 4005cd: 48 89 c1 mov %rax,%rcx
>>> 4005d0: 48 83 f9 03 cmp $0x3,%rcx
>>> 4005d4: 75 60 jne 400636 <main+0x76>
>>> 4005d6: 48 8d 35 89 01 00 00 lea 0x189(%rip),%rsi
>>> # 400766 <_IO_stdin_used+0xe>
>>> 4005dd: 48 89 df mov %rbx,%rdi
>>> 4005e0: 48 89 ca mov %rcx,%rdx
>>> 4005e3: e8 d8 fe ff ff callq 4004c0 <memcmp at plt>
>>> 4005e8: 89 c1 mov %eax,%ecx
>>> 4005ea: b8 1f 00 00 00 mov $0x1f,%eax
>>> 4005ef: 85 c9 test %ecx,%ecx
>>> 4005f1: 74 6f je 400662 <main+0xa2>
>>> 4005f3: 48 8d 35 70 01 00 00 lea 0x170(%rip),%rsi
>>> # 40076a <_IO_stdin_used+0x12>
>>> 4005fa: 48 89 df mov %rbx,%rdi
>>> 4005fd: ba 03 00 00 00 mov $0x3,%edx
>>> 400602: e8 b9 fe ff ff callq 4004c0 <memcmp at plt>
>>> 400607: 89 c1 mov %eax,%ecx
>>> 400609: b8 47 00 00 00 mov $0x47,%eax
>>> 40060e: 85 c9 test %ecx,%ecx
>>> 400610: 74 50 je 400662 <main+0xa2>
>>> 400612: 48 8d 35 55 01 00 00 lea 0x155(%rip),%rsi
>>> # 40076e <_IO_stdin_used+0x16>
>>> 400619: 48 89 df mov %rbx,%rdi
>>> 40061c: ba 03 00 00 00 mov $0x3,%edx
>>> 400621: e8 9a fe ff ff callq 4004c0 <memcmp at plt>
>>> 400626: 89 c1 mov %eax,%ecx
>>> 400628: ba 61 00 00 00 mov $0x61,%edx
>>> 40062d: 31 c0 xor %eax,%eax
>>>
>>> I do notice that the compiler tries to be smart about the length of
>>> the string, but for strings of the same length it has no choice but
>>> sequentially "memcmp" them. Note that the vast majority of assembly
>>> directives fall into very few bins in terms of string length.
>>>
>>> Eli
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