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Metadce fixes (emscripten-core#6444)
 * we cannot metadce linkable code, like shared modules. Fixes emscripten-core#6362 * improve the error message for when the metadce passes cannot find necessary things due to improper use of --pre/--post-js emscripten-core#6442 * we also cannot jsdce linkable code
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Lines changed: 40 additions & 28 deletions

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‎site/source/docs/tools_reference/emcc.rst‎

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@@ -255,7 +255,7 @@ Options that are modified or new in *emcc* are listed below:
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.. _emcc-pre-js:
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``--pre-js <file>``
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Specify a file whose contents are added before the emitted code and optimized together with it. Note that this might not literally be the very first thing in the JS output, for example if ``MODULARIZE`` is used (see ``src/settings.js``). If you want that, you can just prepend to the output from emscripten; the benefit of ``--pre-js`` is that it optimizes the code with the rest of the emscripten output, which allows better dead code elimination and minification.
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Specify a file whose contents are added before the emitted code and optimized together with it. Note that this might not literally be the very first thing in the JS output, for example if ``MODULARIZE`` is used (see ``src/settings.js``). If you want that, you can just prepend to the output from emscripten; the benefit of ``--pre-js`` is that it optimizes the code with the rest of the emscripten output, which allows better dead code elimination and minification, and it should only be used for that purpose. In particular, ``--pre-js`` code should not alter the main output from emscripten in ways that could confuse the optimizer, such as using ``--pre-js`` + ``--post-js`` to put all the output in an inner function scope (see ``MODULARIZE`` for that).
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.. _emcc-post-js:
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‎tests/test_other.py‎

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@@ -7854,15 +7854,18 @@ def test(filename, expectations):
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print('test on hello world')
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test(path_from_root('tests', 'hello_world.cpp'), [
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([], 24, ['abort', 'tempDoublePtr'], ['waka'], 46505, 25, 19),
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(['-O1'], 19, ['abort', 'tempDoublePtr'], ['waka'], 12630, 16, 17),
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(['-O2'], 19, ['abort', 'tempDoublePtr'], ['waka'], 12616, 16, 17),
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(['-O3'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2818, 10, 2), # in -O3, -Os and -Oz we metadce
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(['-Os'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2771, 10, 2),
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(['-Oz'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2765, 10, 2),
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([], 24, ['abort', 'tempDoublePtr'], ['waka'], 46505, 25, 19),
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(['-O1'], 19, ['abort', 'tempDoublePtr'], ['waka'], 12630, 16, 17),
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(['-O2'], 19, ['abort', 'tempDoublePtr'], ['waka'], 12616, 16, 17),
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(['-O3'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2818, 10, 2), # in -O3, -Os and -Oz we metadce
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(['-Os'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2771, 10, 2),
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(['-Oz'], 7, ['abort'], ['tempDoublePtr', 'waka'], 2765, 10, 2),
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# finally, check what happens when we export nothing. wasm should be almost empty
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(['-Os', '-s', 'EXPORTED_FUNCTIONS=[]'],
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0, [], ['tempDoublePtr', 'waka'], 8, 0, 0), # totally empty!
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0, [], ['tempDoublePtr', 'waka'], 8, 0, 0), # totally empty!
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# but we don't metadce with linkable code! other modules may want it
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(['-O3', '-s', 'MAIN_MODULE=1'],
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1542, ['invoke_i'], ['waka'], 496958, 168, 2558),
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])
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print('test on a minimal pure computational thing')

‎tools/js-optimizer.js‎

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@@ -8139,7 +8139,8 @@ function emitDCEGraph(ast) {
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return null; // don't look inside
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}
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});
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assert(foundAsmLibraryArgAssign); // must find the info we need
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// must find the info we need
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assert(foundAsmLibraryArgAssign, 'could not find the assigment to "asmLibraryArg". perhaps --pre-js or --post-js code moved it out of the global scope? (things like that should be done after emcc runs, as they do not need to be run through the optimizer which is the special thing about --pre-js/--post-js code)');
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// Second pass: everything used in the toplevel scope is rooted;
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// things used in defun scopes create links
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function getGraphName(name, what) {

‎tools/shared.py‎

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@@ -2019,6 +2019,7 @@ def emscripten(filename, append_ext=True, extra_args=[]):
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return filename + '.o.js'
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# TODO: deprecate this method, we should just need Settings.LINKABLE anyhow
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@staticmethod
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def can_build_standalone():
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return not Settings.BUILD_AS_SHARED_LIB and not Settings.LINKABLE
@@ -2217,27 +2218,34 @@ def closure_compiler(filename, pretty=True):
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@staticmethod
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def minify_wasm_js(js_file, wasm_file, expensive_optimizations, minify_whitespace, use_closure_compiler, debug_info, emit_symbol_map):
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# start with JSDCE, to clean up obvious JS garbage. When optimizing for size,
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# use AJSDCE (aggressive JS DCE, performs multiple iterations)
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passes = ['noPrintMetadata', 'JSDCE' if not expensive_optimizations else 'AJSDCE']
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# use AJSDCE (aggressive JS DCE, performs multiple iterations). Clean up
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# whitespace if necessary too.
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passes = []
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if not Settings.LINKABLE:
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passes.append('JSDCE' if not expensive_optimizations else 'AJSDCE')
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if minify_whitespace:
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passes.append('minifyWhitespace')
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logging.debug('running cleanup on shell code: ' + ' '.join(passes))
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js_file = Building.js_optimizer_no_asmjs(js_file, passes)
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# if we are optimizing for size, shrink the combined wasm+JS
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# TODO: support this when a symbol map is used
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if expensive_optimizations and not emit_symbol_map:
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js_file = Building.metadce(js_file, wasm_file, minify_whitespace=minify_whitespace, debug_info=debug_info)
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# now that we removed unneeded communication between js and wasm, we can clean up
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# the js some more.
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passes = ['noPrintMetadata', 'AJSDCE']
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if minify_whitespace:
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passes.append('minifyWhitespace')
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logging.debug('running post-meta-DCE cleanup on shell code: ' + ' '.join(passes))
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js_file = Building.js_optimizer_no_asmjs(js_file, passes)
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# finally, optionally use closure compiler to finish cleaning up the JS
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if use_closure_compiler:
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logging.debug('running closure on shell code')
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js_file = Building.closure_compiler(js_file, pretty=not minify_whitespace)
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if passes:
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logging.debug('running cleanup on shell code: ' + ' '.join(passes))
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js_file = Building.js_optimizer_no_asmjs(js_file, ['noPrintMetadata'] + passes)
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# if we can optimize this js+wasm combination under the assumption no one else
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# will see the internals, do so
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if not Settings.LINKABLE:
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# if we are optimizing for size, shrink the combined wasm+JS
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# TODO: support this when a symbol map is used
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if expensive_optimizations and not emit_symbol_map:
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js_file = Building.metadce(js_file, wasm_file, minify_whitespace=minify_whitespace, debug_info=debug_info)
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# now that we removed unneeded communication between js and wasm, we can clean up
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# the js some more.
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passes = ['noPrintMetadata', 'AJSDCE']
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if minify_whitespace:
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passes.append('minifyWhitespace')
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logging.debug('running post-meta-DCE cleanup on shell code: ' + ' '.join(passes))
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js_file = Building.js_optimizer_no_asmjs(js_file, passes)
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# finally, optionally use closure compiler to finish cleaning up the JS
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if use_closure_compiler:
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logging.debug('running closure on shell code')
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js_file = Building.closure_compiler(js_file, pretty=not minify_whitespace)
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return js_file
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# run binaryen's wasm-metadce to dce both js and wasm

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