[LLVMbugs] [Bug 19217] New: Extremely slow compilation due to greedy register allocator

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Thu Mar 20 17:57:09 PDT 2014


            Bug ID: 19217
           Summary: Extremely slow compilation due to greedy register
           Product: clang
           Version: 3.4
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: feeley at iro.umontreal.ca
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The compilation of a large C file (generated by the Gambit Scheme to C
compiler) is very slow at -O1 and a time-report indicates that 97% of the time
is spent in the greedy register allocator.  The file takes 3000 times longer to
compile with -O1 than with -O0 (35 minutes vs. 0.7 seconds).

The issue can be reproduced with this invocation of clang:

 clang -O1 -c -D___SINGLE_HOST _x86.c

Note that gcc-4.8.2 takes 5 seconds to compile with the same options.

The clang I use is Apple's LLVM version 5.1 in the latest Xcode:

% clang -v
Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix

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