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[WebAssembly] Elide data segments for .bss sections

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Summary:
WebAssembly memories are zero-initialized, so when module does not
import its memory initializing .bss sections is guaranteed to be a
no-op. To reduce binary size and initialization time, .bss sections
are simply not emitted into the final binary unless the memory is
imported.

Reviewers: sbc100

Subscribers: dschuff, jgravelle-google, aheejin, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68965

llvm-svn: 374940
This commit is contained in:
Thomas Lively
2019-10-15 19:05:11 +00:00
parent 0330fba6e1
commit 190dacc3cc
13 changed files with 75 additions and 49 deletions
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9
lld/wasm/Writer.cpp
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@@ -669,6 +669,13 @@ void Writer::createOutputSegments() {
s = make<OutputSegment>(name);
if (config->sharedMemory || name == ".tdata")
s->initFlags = WASM_SEGMENT_IS_PASSIVE;
// Exported memories are guaranteed to be zero-initialized, so no need
// to emit data segments for bss sections.
// TODO: consider initializing bss sections with memory.fill
// instructions when memory is imported and bulk-memory is available.
if (!config->importMemory && !config->relocatable &&
name.startswith(".bss"))
s->isBss = true;
segments.push_back(s);
}
s->addInputSegment(segment);
@@ -961,7 +968,7 @@ void Writer::createSyntheticSections() {
out.exportSec = make<ExportSection>();
out.startSec = make<StartSection>(segments.size());
out.elemSec = make<ElemSection>();
out.dataCountSec = make<DataCountSection>(segments.size());
out.dataCountSec = make<DataCountSection>(segments);
out.linkingSec = make<LinkingSection>(initFunctions, segments);
out.nameSec = make<NameSection>();
out.producersSec = make<ProducersSection>();