5 ms·

Another interesting point is that, only gcc produced binary has that behavior. clang generated is the opposite. Check this comment about testing on Sandy Bridge

by songgao 13y ago

Another interesting point is that, only gcc produced binary has that behavior. clang generated is the opposite. Check this comment about testing on Sandy Bridge with gcc and clang: http://eli.thegreenplace.net/2013/12/03/intel-i7-loop-performance-anomaly/#comment-1298089 http://eli.thegreenplace.net/2013/12/03/intel-i7-loop-perfor...

abcd_f 13y ago
This begs for comparing assembly dumps of both.
- ksherlock 13y ago
  gcc acts RISCy -- load memory into a register, add registers, store register back to memory. clang acts CISCy -- use a read-modify-write instruction to add directly in memory.
  - rayiner 13y ago
    I'm surprised that GCC generates that code. Using read-modify-write instructions directly in memory has been better on x86 processors for quite awhile now, since both AMD and Intel support fusing the memory and arithmetic operations into a single micro-op.
    - eliben 13y ago
      I was studying this difference between the two compilers for a different issue, actually. According to the standard, I don't see anything that prevents an increment of a volatile to be performed in a single instruction if the target permits it. Splitting it to a separate load+binop+store by gcc seems like an over-pessimization.
    - pbsd 13y ago
      Curiously enough, using separate reads and writes, with appropriate nop padding, seems to do better than any read-modify-write variation I tried. By the way, read-modify-write is not fused altogether: only the write part is fused, so it generates 2 unfused uops (read, add), plus the fused uop for the write + address generation. EDIT: Intel's compiler also generates read-modify-write code, but it ends up being slower than both gcc and clang.
      - nkurz 13y ago
        I also have gotten better results with separate read, modify, and write instructions than with combined. I think this is because the separate assembly statements allow for more explicit scheduling.

by songgao 13y ago

abcd_f 13y ago

This begs for comparing assembly dumps of both.

ksherlock 13y ago
gcc acts RISCy -- load memory into a register, add registers, store register back to memory. clang acts CISCy -- use a read-modify-write instruction to add directly in memory.
- rayiner 13y ago
  I'm surprised that GCC generates that code. Using read-modify-write instructions directly in memory has been better on x86 processors for quite awhile now, since both AMD and Intel support fusing the memory and arithmetic operations into a single micro-op.
  - eliben 13y ago
    I was studying this difference between the two compilers for a different issue, actually. According to the standard, I don't see anything that prevents an increment of a volatile to be performed in a single instruction if the target permits it. Splitting it to a separate load+binop+store by gcc seems like an over-pessimization.
  - pbsd 13y ago
    Curiously enough, using separate reads and writes, with appropriate nop padding, seems to do better than any read-modify-write variation I tried. By the way, read-modify-write is not fused altogether: only the write part is fused, so it generates 2 unfused uops (read, add), plus the fused uop for the write + address generation. EDIT: Intel's compiler also generates read-modify-write code, but it ends up being slower than both gcc and clang.
    - nkurz 13y ago
      I also have gotten better results with separate read, modify, and write instructions than with combined. I think this is because the separate assembly statements allow for more explicit scheduling.