Ok, I understand better.
What makes things confusing is probably more that the usual distinction
between compiler flags and linker flags are a bit blurred in Emscripten.
As "WASM_OBJECT_FILES=0" is doing the same as "-flto", but also applying
it to system libs, maybe it would be cleaner to have it replaced by a
"-s ENABLE_SYSTEM_LIBS_LTO=1" that would only focus on enabling LTO on
the system libs (letting the user control the LTO of its own code with
the usual -flto flag).
Btw, the docs mention that --lvm-lto might have the values 1,2 or 3 =>
is this still applicable to the new Wasm backend ? I guess no (?)
Cheers,
Gabriel
Le 22/10/2019 à 21:39, Alon Zakai a écrit :
The LTO docs are here:
https://emscripten.org/docs/optimizing/Optimizing-Code.html#lto
Basically
* -flto is a clang flag that says "emit LLVM IR in the bitcode file"
* WASM_OBJECT_FILES=0 is an emscripten flag that says "don't use wasm
object files, so use LLVM IR in bitcode files, *and also in system libs*"
* --llvm-lto 1 is an emscripten flag that also runs LLVM's default
LTO opts at link time.
Perhaps -flto should automatically set --llvm-lto 1? I'm not sure what
clang normally does, but we should do the same probably...
- Alon
On Tue, Oct 22, 2019 at 2:40 AM Gabriel Cuvillier
<[email protected] <mailto:[email protected]>> wrote:
Great!
A question regarding this in the documentation, which confuses me
a bit:
/You can disable wasm object files with -s WASM_OBJECT_FILES=0,
which will make the wasm backend behave more like fastcomp. //
//Neither fastcomp nor the wasm backend without wasm object files
will run the LLVM optimization passes by default, even if using
LLVM IR in object files; //
//for that you must pass --llvm-lto 1.//
/
With the 3 options "-s WASM_OBJECT_FILES=0", "--llvm-lto 1", and
"-flto", there is a lot of combinations possible and the outcome
is not quite clear.
So, on the Wasm backed, what's the difference between doing (or
what is meaningful/meaningless):
-flto
--llvm-lto 1
-s WASM_OBJECT_FILES=0
-flto --llvm-lto 1
-flto -s WASM_OBJECT_FILES=0
-flto -s WASM_OBJECT_FILES=0 --llvm-lto 1
-s WASM_OBJECT_FILES=0 --llvm-lto 1
+ Which option shall be passed at compilation time / at link time ?
Thanks!
Le 22/10/2019 à 00:17, Alon Zakai a écrit :
Hello everyone,
The emsdk will now install the upstream LLVM backend by default,
instead of fastcomp, as of
https://github.com/emscripten-core/emsdk/pull/373
That is, if you install/activate "latest" then you get the same
as "latest-upstream". It used to be an alias for "latest-fastcomp".
You can still use "latest-fastcomp" to get fastcomp if you need
that. You can also install stuff like "1.39.0-fastcomp", a
version with a specific backend.
The emsdk defaults to the upstream backend from 1.39.0, but not
earlier versions, as the upstream backend wasn't stable enough
yet at those times. (But you can as always do "1.38.39-upstream"
to get upstream for those versions.) In other words, the only
thing that changed now is that for 1.39.0 and onwards, if you
don't specify the backend, you'll get upstream.
We are changing the default now after a long period of
recommending people upgrade to the LLVM backend and fixing issues
based on their feedback, as a result of which at this point we
feel comfortable changing the default.
Please test and file bugs if you find any! All you need to do is
update to the latest emsdk master from github, and
./emsdk install latest
./emsdk activate latest
If this is your first time using the upstream backend, see the
list of differences,
https://emscripten.org/docs/compiling/WebAssembly.html#backends
Our goal is to remove fastcomp eventually, and it is now
officially deprecated, but we will only start to plan that after
seeing the feedback from users after this switch.
- Alon
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