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GCC 8 Link Time and Interprocedural Optimization

143 points by edelsohn 8y ago

ezoe 8y ago
>so you can rebuild binary with debug info after your program core dumped and use it to debug the core dump I've never thought it's possible. Rebuild after the core dump ? Rather than build with debug info and strip it away.
- ebikelaw 8y ago
  If your build system is reproducible there is no reason this would not work.
  - jlarocco 8y ago
    Maybe that's the case now, with GCC 8, but historically that hasn't been true. In the past the effects of using both -g and -O* flags could interact each other because they can both affect how registers are allocated, how call stacks are setup, how functions are inlined, etc. In other words, adding -g by itself made the build non-reproducible. If that's no longer true in GCC 8 it's great news.
    - glandium 8y ago
      > In other words, adding -g by itself made the build non-reproducible. In my experience, this is largely not true. However, what is plausible is that before GCC 8, LTO, which, for speed reasons, can split work across multiple threads, was not deterministic, and GCC 8 might have made that better.
    - readittwice 8y ago
      AFAIR GCC bootstrap builds by default with "-g -O2". There are 3 stages: 1) use system compiler to compile GCC's source code, 2) use resulting binary to build same source, 3) use binary from 2) to build source one more time. After all stages the binaries from stage 2 and 3 are compared byte for byte if they are identical. Bootstrap failures are taken very seriously by the GCC developers. So I really doubt that -g and -O* lead to non-deterministic builds.
      - deleted 8y ago
        [deleted]
bumblebritches5 8y ago
The community would be better off by dumping gcc's dead ass, and supporting clang and LLVM entirely.
plmkijnhyt 8y ago
this is a very nice forum,check it out https://www.bit.ly/2M0hqJa https://www.bit.ly/2M0hqJa
42432534532532 8y ago
this is a very nice forum,check it out https://www.bit.ly/2M0hqJa https://www.bit.ly/2M0hqJa
geocar 8y ago
Static cold detection sounds really useful. How many __builtin_expects can it remove?
tylll 8y ago
nice post https://bit.ly/2HVeGKU https://bit.ly/2HVeGKU
uwu 8y ago
when i was a full-time hobbyist webdev i was surprised when i learned c compilers don't do whole-program optimization by default but limit themselves by optimizing each translation unit separately (i guess it makes things simpler) you can get "LTO" in javascript by using a minifier together with something like webpack which can inline modules when it's safe to do so (instead of putting them in functions that are called by the module loader and can't be minified away), so i assumed "real" compilers must do that stuff a lot better with the language being less dynamic
- srean 8y ago
  In addition to simplicity it cuts down on compilation time. You can checkout MLTon or Felix to get a feel for whole program optimizing compilers, they can do optimizations that are impossible without LTO and hard to do with. https://github.com/felix-lang/felix https://github.com/felix-lang/felix http://felix-documentation-master.readthedocs.io/en/latest/masterdirectory.html#specific-documents http://felix-documentation-master.readthedocs.io/en/latest/m... http://mlton.org http://mlton.org

143 points by edelsohn 8y ago

ezoe 8y ago

>so you can rebuild binary with debug info after your program core dumped and use it to debug the core dump I've never thought it's possible. Rebuild after the core dump ? Rather than build with debug info and strip it away.

ebikelaw 8y ago
If your build system is reproducible there is no reason this would not work.
- jlarocco 8y ago
  Maybe that's the case now, with GCC 8, but historically that hasn't been true. In the past the effects of using both -g and -O* flags could interact each other because they can both affect how registers are allocated, how call stacks are setup, how functions are inlined, etc. In other words, adding -g by itself made the build non-reproducible. If that's no longer true in GCC 8 it's great news.
  - glandium 8y ago
    > In other words, adding -g by itself made the build non-reproducible. In my experience, this is largely not true. However, what is plausible is that before GCC 8, LTO, which, for speed reasons, can split work across multiple threads, was not deterministic, and GCC 8 might have made that better.
  - readittwice 8y ago
    AFAIR GCC bootstrap builds by default with "-g -O2". There are 3 stages: 1) use system compiler to compile GCC's source code, 2) use resulting binary to build same source, 3) use binary from 2) to build source one more time. After all stages the binaries from stage 2 and 3 are compared byte for byte if they are identical. Bootstrap failures are taken very seriously by the GCC developers. So I really doubt that -g and -O* lead to non-deterministic builds.
    - deleted 8y ago
      [deleted]

bumblebritches5 8y ago

The community would be better off by dumping gcc's dead ass, and supporting clang and LLVM entirely.

plmkijnhyt 8y ago

this is a very nice forum,check it out https://www.bit.ly/2M0hqJa https://www.bit.ly/2M0hqJa

42432534532532 8y ago

this is a very nice forum,check it out https://www.bit.ly/2M0hqJa https://www.bit.ly/2M0hqJa

geocar 8y ago

Static cold detection sounds really useful. How many __builtin_expects can it remove?

tylll 8y ago

nice post https://bit.ly/2HVeGKU https://bit.ly/2HVeGKU

uwu 8y ago

when i was a full-time hobbyist webdev i was surprised when i learned c compilers don't do whole-program optimization by default but limit themselves by optimizing each translation unit separately (i guess it makes things simpler) you can get "LTO" in javascript by using a minifier together with something like webpack which can inline modules when it's safe to do so (instead of putting them in functions that are called by the module loader and can't be minified away), so i assumed "real" compilers must do that stuff a lot better with the language being less dynamic

srean 8y ago
In addition to simplicity it cuts down on compilation time. You can checkout MLTon or Felix to get a feel for whole program optimizing compilers, they can do optimizations that are impossible without LTO and hard to do with. https://github.com/felix-lang/felix https://github.com/felix-lang/felix http://felix-documentation-master.readthedocs.io/en/latest/masterdirectory.html#specific-documents http://felix-documentation-master.readthedocs.io/en/latest/m... http://mlton.org http://mlton.org