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
Summary: Some frontends emit a speculate-and-select idiom for sqrt, wherein they compute sqrt(x), check if x is negative, and select NaN if it is: %cmp = fcmp olt double %a, -0.000000e+00 %sqrt = call double @llvm.sqrt.f64(double %a) %ret = select i1 %cmp, double 0x7FF8000000000000, double %sqrt This is technically UB as the LangRef is written today if %a is ever less than -0. But emitting code that's compliant with the current definition of sqrt would require a branch, which would then prevent us from matching this idiom in SelectionDAG (which we do today -- ISD::FSQRT has defined behavior on negative inputs), because SelectionDAG looks at one BB at a time. Nothing in LLVM takes advantage of this undefined behavior, as far as we can tell, and the fact that llvm.sqrt has UB dates from its initial addition to the LangRef. Reviewers: arsenm, mehdi_amini, hfinkel Subscribers: wdng, llvm-commits Differential Revision: https://reviews.llvm.org/D28797 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293242 91177308-0d34-0410-b5e6-96231b3b80d8
LLVM Documentation ================== LLVM's documentation is written in reStructuredText, a lightweight plaintext markup language (file extension `.rst`). While the reStructuredText documentation should be quite readable in source form, it is mostly meant to be processed by the Sphinx documentation generation system to create HTML pages which are hosted on <http://llvm.org/docs/> and updated after every commit. Manpage output is also supported, see below. If you instead would like to generate and view the HTML locally, install Sphinx <http://sphinx-doc.org/> and then do: cd <build-dir> cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_HTML=true <src-dir> make -j3 docs-llvm-html $BROWSER <build-dir>/docs//html/index.html The mapping between reStructuredText files and generated documentation is `docs/Foo.rst` <-> `<build-dir>/docs//html/Foo.html` <-> `http://llvm.org/docs/Foo.html`. If you are interested in writing new documentation, you will want to read `SphinxQuickstartTemplate.rst` which will get you writing documentation very fast and includes examples of the most important reStructuredText markup syntax. Manpage Output =============== Building the manpages is similar to building the HTML documentation. The primary difference is to use the `man` makefile target, instead of the default (which is `html`). Sphinx then produces the man pages in the directory `<build-dir>/docs/man/`. cd <build-dir> cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_MAN=true <src-dir> make -j3 docs-llvm-man man -l >build-dir>/docs/man/FileCheck.1 The correspondence between .rst files and man pages is `docs/CommandGuide/Foo.rst` <-> `<build-dir>/docs//man/Foo.1`. These .rst files are also included during HTML generation so they are also viewable online (as noted above) at e.g. `http://llvm.org/docs/CommandGuide/Foo.html`. Checking links ============== The reachability of external links in the documentation can be checked by running: cd docs/ make -f Makefile.sphinx linkcheck