38 lines
1.5 KiB
Plaintext
38 lines
1.5 KiB
Plaintext
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smalluint i = index_in_str_array(params, name) + 1;
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if (i == 0)
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return 0;
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if (!(i == 4 || i == 5))
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i |= 0x80;
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return i;
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I think that this optimization is wrong.
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index_in_str_array returns int. At best, compiler will use it as-is.
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At worst, compiler will try to make sure that it is properly casted
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into a byte, which probably results in "n = n & 0xff" on many architectures.
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You save nothing on space here because i is not stored on-stack,
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gcc will keep it in register. And even it is *is* stored,
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it is *stack* storage, which is cheap (unlike data/bss).
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small[u]ints are useful _mostly_ for:
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(a) flag variables
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(a1) global flag variables - make data/bss smaller
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(a2) local flag variables - "a = 5", "a |= 0x40" are smaller
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for bytes than for full integers.
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Example:
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on i386, there is no widening constant store instruction
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for some types of address modes, thus
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movl $0x0,(%eax) is "c7 00 00 00 00 00"
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movb $0x0,(%eax) is "c6 00 00"
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(b) small integer structure members, when you have many such
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structures allocated,
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or when these are global objects of this structure type
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small[u]ints are *NOT* useful for:
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(a) function parameters and return values -
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they are pushed on-stack or stored in registers, bytes here are *harder*
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to deal with than ints
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(b) "computational" variables - "a++", "a = b*3 + 7" may take more code to do
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on bytes than on ints on some architectires.
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