<div dir="ltr"><div dir="ltr"><p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Hi,</p>
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<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Bruno (CCed), Duncan (CCed) and I have been exploring if we can migrate some of our clients to explicit modules. As part of this work Duncan and I developed a new prototype dependency scanning service tool (clang-scan-deps) that computes the set of file dependencies for a particular compiler invocation using some optimizations that are outlined below. This tool makes the non-modular dependency scanning up to 10 times faster for particular workloads (e.g. llc target, 1542 C++ files) on one of our machines, when compared to parallel invocations of clang with -Eonly. We are still in the early stages of proper modules support, but our initial crude prototype can get up to 4x when run on the first 1000 files from clang’s compilation database for a build of LLVM with modules turned on.</p>
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<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">We still run the full Clang preprocessor. Here’s what we do to reduce its workload:</p>
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<li style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Minimize sources by stripping away unused tokens. We keep only the interesting PP directives (#define, #if, #include, etc.), i.e. those that might impact the set of dependencies. </li>
<li style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Assume the filesystem is immutable for one run of the service, and cache the files and their minimized contents in memory in a global cache.</li>
<li style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Skip over excluded preprocessor ranges by bumping up the buffer pointer in the lexer instead of lexing the skipped tokens.</li>
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<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">We intend to upstream this service in the upcoming months. We also would like to integrate this service into Clangd as part of our migration to Clangd to help us determine a good compilation command for a header file from a set of known compilation invocations.</p>
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<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">I posted a very rough WIP patch on Phabricator (<a href="https://reviews.llvm.org/D53354"><span style="color:rgb(220,161,13)">https://reviews.llvm.org/D53354</span></a>). It’s based on LLVM checkout r343343. Please take a look if you’re interested.</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Duncan, Bruno and I will be at the LLVM dev meeting. We are interested in discussing this prototype and collecting feedback from anyone who might be interested in this work.</p>
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<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Thanks,</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:"Helvetica Neue"">Alex</p></div></div>