3 ms·

Why couldn't they though? Doesn't sound very hard to only generate the wordy prologues and epilogues when necessary (i.e. when you have to save any registers).

by aap_ 10y ago

Why couldn't they though? Doesn't sound very hard to only generate the wordy prologues and epilogues when necessary (i.e. when you have to save any registers). Why they apparently don't do this is another question then.

Annatar 10y ago
I'm not aware of a general algorithm which is capable of deciding whether and how many processor's registers to use instead of setting up frame and stack pointers, and pushing a variable number of arguments on the stack, are you? A human will know while coding in assembler, at any given time, how many variables are in the game; and will almost always manage to fit them all within processor's registers; There were only two times in my life where I actually had more than eight variables within a subroutine and had to use the stack, and even then, I didn't push everything, but only as many registers as I was actually coming up short, and the rest I still stuffed in the available registers. The other time, I figured out a more efficient algorithm where I could fit everything within the seven general purpose address registers (a0 - a6, since a7 is the stack pointer). A human will also know whether the expected result is within a byte, word, longword, or quadword range, and will only use those instruction and register sizes; a compiler has no chance to figure that out. It's trivial for humans, but as far as I'm aware, impossible for a general compiler algorithm. In fact, even the best optimizing compilers are so dumb, that one is not allowed to mix and match 8-, 16-, 32- or 64-bit code; one must either compile everything 32- or 64-bit (the linker won't let one link 32- and 64-bit object code together). A human could easily write correct assembler code using all of those instruction / register sizes at once, and we often do. I have yet to see a compiler capable of inferring that. If you know of one, please show me the generated code. I'd love to use such a compiler.

by aap_ 10y ago

Annatar 10y ago

I'm not aware of a general algorithm which is capable of deciding whether and how many processor's registers to use instead of setting up frame and stack pointers, and pushing a variable number of arguments on the stack, are you? A human will know while coding in assembler, at any given time, how many variables are in the game; and will almost always manage to fit them all within processor's registers; There were only two times in my life where I actually had more than eight variables within a subroutine and had to use the stack, and even then, I didn't push everything, but only as many registers as I was actually coming up short, and the rest I still stuffed in the available registers. The other time, I figured out a more efficient algorithm where I could fit everything within the seven general purpose address registers (a0 - a6, since a7 is the stack pointer). A human will also know whether the expected result is within a byte, word, longword, or quadword range, and will only use those instruction and register sizes; a compiler has no chance to figure that out. It's trivial for humans, but as far as I'm aware, impossible for a general compiler algorithm. In fact, even the best optimizing compilers are so dumb, that one is not allowed to mix and match 8-, 16-, 32- or 64-bit code; one must either compile everything 32- or 64-bit (the linker won't let one link 32- and 64-bit object code together). A human could easily write correct assembler code using all of those instruction / register sizes at once, and we often do. I have yet to see a compiler capable of inferring that. If you know of one, please show me the generated code. I'd love to use such a compiler.