Neil Hodgson added the comment:
For 32-bit Windows, the code generated for unicode_compare is quite slow.
There are either 1 or 2 kind checks in each call to PyUnicode_READ and 2
calls to PyUnicode_READ inside the loop. A compiler may decide to move the kind
checks out of the loop and specialize the loop but MSVC 2010 appears to not do
so. The assembler (32-bit build) for each PyUnicode_READ looks like
mov ecx, DWORD PTR _kind1$[ebp]
cmp ecx, 1
jne SHORT $LN17@unicode_co@2
lea ecx, DWORD PTR [ebx+eax]
movzx edx, BYTE PTR [ecx+edx]
jmp SHORT $LN16@unicode_co@2
$LN17@unicode_co@2:
cmp ecx, 2
jne SHORT $LN15@unicode_co@2
movzx edx, WORD PTR [ebx+edi]
jmp SHORT $LN16@unicode_co@2
$LN15@unicode_co@2:
mov edx, DWORD PTR [ebx+esi]
$LN16@unicode_co@2:
The kind1/kind2 variables aren't even going into registers and at least one
test+branch and a jump are executed for every character. Two tests for 2 and 4
byte kinds. len1 and len2 don't get to go into registers either.
My system isn't set up for 64-bit MSVC 2010 but looking at the code from
64-bit MSVC 2012 shows that all the variables have been moved into registers
but the kind checking is still inside the loop. This accounts for better
results with 64-bit Python 3.3 on Windows but isn't as good as Unix or Python
3.2.
; 10431: c1 = PyUnicode_READ(kind1, data1, i);
cmp rsi, 1
jne SHORT $LN17@unicode_co
lea rax, QWORD PTR [r9+rcx]
movzx r8d, BYTE PTR [rax+rbx]
jmp SHORT $LN16@unicode_co
$LN17@unicode_co:
cmp rsi, 2
jne SHORT $LN15@unicode_co
movzx r8d, WORD PTR [r9+r11]
jmp SHORT $LN16@unicode_co
$LN15@unicode_co:
mov r8d, DWORD PTR [r9+r10]
$LN16@unicode_co:
Attached the 32-bit assembler listing.
----------
Added file: http://bugs.python.org/file29673/unicode_compare.asm
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<http://bugs.python.org/issue17615>
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