[LLVMbugs] [Bug 16756] New: Extremely slow compilation of innocuous C++ source

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Jul 30 15:01:31 PDT 2013


            Bug ID: 16756
           Summary: Extremely slow compilation of innocuous C++ source
           Product: clang
           Version: 3.2
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jrk at csail.mit.edu
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 10959
  --> http://llvm.org/bugs/attachment.cgi?id=10959&action=edit
Source file which causes slow compilation performance

Halide is a not-especially-complicated C++ project (which happens to use LLVM
internally, but that's not the subject of this bug):


Its build process is simple. On most platforms, GCC and MSVC toolchains compile
of the C++ source quickly. On OS X (10.8), the stock g++ (llvm-gcc-4.2-based)
compiles all but one source file quickly. But on CodeGen_ARM.cpp
(https://github.com/halide/Halide/blob/master/src/CodeGen_ARM.cpp), it is
pathologically slow (>7 minutes on a 2.8ghz Core2 Xeon Mac Pro, ~5 on a
Sandybridge MacBook Air). The same file compiles in the expected second or
three on any Homebrew GCC version, using the full GNU toolchain.

Clearly, this tickles something serious in the Apple/LLVM toolchain. The
process which chugs for minutes during this is named "clang". The only obvious
potential standout here is the relatively complex stack-allocated array
"patterns" in CodeGen_ARM::visit(const Cast *op).

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