4 ms·

I believe adc is still commonly used as a way of reading the carry bit. For example, assuming eax has already been zeroed, adc eax, eax would be the idiomatic w

I believe adc is still commonly used as a way of reading the carry bit. For example, assuming eax has already been zeroed, adc eax, eax would be the idiomatic way of reading the carry bit into eax. The carry flag gets set by all sorts of instructions other than addition and subtraction (comparisons, bit shifts, multiplication, etc) so this is more useful than you might think.

giovannibajo1 6y ago
Actually, setc is the idiomatic way, and doesn’t depend on the previous value of the destination register so it’s much faster to execute because it has no dependencies
- zwegner 6y ago
  No, setcc actually does depend on the previous value of the register, because it only comes in the low-byte variants (i.e. al, bl, cl, dl etc)--the top 7 bytes of the destination are left unmodified. A one-instruction variant I've seen that gets you the negated value of the carry flag is sbb rax, rax. This doesn't depend on the previous value of rax in a mathematical sense, though I'm unsure if it depends in an out-of-order sense; that is, if it's recognized as a zeroing idiom (or rather, zeroing or all-one-ing in this case). Probably not. EDIT: did a quick test, and sbb rax, rax etc are not recognized to be a zeroing idiom, at least on my old Haswell. So it's still one instruction, but has a dependency on the previous register value.
  - nayuki 6y ago
    > setcc actually does depend on the previous value of the register, because it only comes in the low-byte variants You should be able to break the dependency and avoid the partial register stall by doing: movzx eax, ax See: https://stackoverflow.com/questions/41573502/why-doesnt-gcc-use-partial-registers https://stackoverflow.com/questions/41573502/why-doesnt-gcc-... ; https://software.intel.com/en-us/forums/intel-isa-extensions/topic/297441 https://software.intel.com/en-us/forums/intel-isa-extensions... ; https://www.agner.org/optimize/microarchitecture.pdf https://www.agner.org/optimize/microarchitecture.pdf (section 6.8)
    - zwegner 6y ago
      You're right, but only for pre-Haswell chips, where al etc. are renamed separately. From Haswell onwards, they aren't, and depend on the old value. So movzx eax, al gets you mathematical independence, but not out-of-order independence. There's a bunch more detail in this Stack Overflow question (referenced in the one you linked): https://stackoverflow.com/questions/45660139/how-exactly-do-partial-registers-on-haswell-skylake-perform-writing-al-seems-to https://stackoverflow.com/questions/45660139/how-exactly-do-...
kccqzy 6y ago
The carry flag is also the canonical way to signal "something has gone wrong" in low-level code. Example: Intel virtual machine extension instructions (like vmwrite, vmxon) all set the carry flag when they fail. But no, you'd usually use jc or setc to read the flag.

giovannibajo1 6y ago

Actually, setc is the idiomatic way, and doesn’t depend on the previous value of the destination register so it’s much faster to execute because it has no dependencies

zwegner 6y ago
No, setcc actually does depend on the previous value of the register, because it only comes in the low-byte variants (i.e. al, bl, cl, dl etc)--the top 7 bytes of the destination are left unmodified. A one-instruction variant I've seen that gets you the negated value of the carry flag is sbb rax, rax. This doesn't depend on the previous value of rax in a mathematical sense, though I'm unsure if it depends in an out-of-order sense; that is, if it's recognized as a zeroing idiom (or rather, zeroing or all-one-ing in this case). Probably not. EDIT: did a quick test, and sbb rax, rax etc are not recognized to be a zeroing idiom, at least on my old Haswell. So it's still one instruction, but has a dependency on the previous register value.
- nayuki 6y ago
  > setcc actually does depend on the previous value of the register, because it only comes in the low-byte variants You should be able to break the dependency and avoid the partial register stall by doing: movzx eax, ax See: https://stackoverflow.com/questions/41573502/why-doesnt-gcc-use-partial-registers https://stackoverflow.com/questions/41573502/why-doesnt-gcc-... ; https://software.intel.com/en-us/forums/intel-isa-extensions/topic/297441 https://software.intel.com/en-us/forums/intel-isa-extensions... ; https://www.agner.org/optimize/microarchitecture.pdf https://www.agner.org/optimize/microarchitecture.pdf (section 6.8)
  - zwegner 6y ago
    You're right, but only for pre-Haswell chips, where al etc. are renamed separately. From Haswell onwards, they aren't, and depend on the old value. So movzx eax, al gets you mathematical independence, but not out-of-order independence. There's a bunch more detail in this Stack Overflow question (referenced in the one you linked): https://stackoverflow.com/questions/45660139/how-exactly-do-partial-registers-on-haswell-skylake-perform-writing-al-seems-to https://stackoverflow.com/questions/45660139/how-exactly-do-...

kccqzy 6y ago

The carry flag is also the canonical way to signal "something has gone wrong" in low-level code. Example: Intel virtual machine extension instructions (like vmwrite, vmxon) all set the carry flag when they fail. But no, you'd usually use jc or setc to read the flag.