src/llvm/tools/clang/NOTES.txt

   1 //===---------------------------------------------------------------------===//
   2 // Random Notes
   3 //===---------------------------------------------------------------------===//
   4
   5 C90/C99/C++ Comparisons:
   6 http://david.tribble.com/text/cdiffs.htm
   7
   8 //===---------------------------------------------------------------------===//
   9
  10 To time GCC preprocessing speed without output, use:
  11    "time gcc -MM file"
  12 This is similar to -Eonly.
  13
  14 //===---------------------------------------------------------------------===//
  15
  16 Creating and using a PTH file for performance measurement (use a release build).
  17
  18 $ clang -ccc-pch-is-pth -x objective-c-header INPUTS/Cocoa_h.m -o /tmp/tokencache
  19 $ clang -cc1 -token-cache /tmp/tokencache INPUTS/Cocoa_h.m
  20
  21 //===---------------------------------------------------------------------===//
  22
  23   C++ Template Instantiation benchmark:
  24      http://users.rcn.com/abrahams/instantiation_speed/index.html
  25
  26 //===---------------------------------------------------------------------===//
  27
  28 TODO: File Manager Speedup:
  29
  30  We currently do a lot of stat'ing for files that don't exist, particularly
  31  when lots of -I paths exist (e.g. see the <iostream> example, check for
  32  failures in stat in FileManager::getFile).  It would be far better to make
  33  the following changes:
  34    1. FileEntry contains a sys::Path instead of a std::string for Name.
  35    2. sys::Path contains timestamp and size, lazily computed.  Eliminate from
  36       FileEntry.
  37    3. File UIDs are created on request, not when files are opened.
  38  These changes make it possible to efficiently have FileEntry objects for
  39  files that exist on the file system, but have not been used yet.
  40
  41  Once this is done:
  42    1. DirectoryEntry gets a boolean value "has read entries".  When false, not
  43       all entries in the directory are in the file mgr, when true, they are.
  44    2. Instead of stat'ing the file in FileManager::getFile, check to see if
  45       the dir has been read.  If so, fail immediately, if not, read the dir,
  46       then retry.
  47    3. Reading the dir uses the getdirentries syscall, creating an FileEntry
  48       for all files found.
  49
  50 //===---------------------------------------------------------------------===//
  51 // Specifying targets:  -triple and -arch
  52 //===---------------------------------------------------------------------===//
  53
  54 The clang supports "-triple" and "-arch" options. At most one -triple and one
  55 -arch option may be specified.  Both are optional.
  56
  57 The "selection of target" behavior is defined as follows:
  58
  59 (1) If the user does not specify -triple, we default to the host triple.
  60 (2) If the user specifies a -arch, that overrides the arch in the host or
  61     specified triple.
  62
  63 //===---------------------------------------------------------------------===//
  64
  65
  66 verifyInputConstraint and verifyOutputConstraint should not return bool.
  67
  68 Instead we should return something like:
  69
  70 enum VerifyConstraintResult {
  71   Valid,
  72
  73   // Output only
  74   OutputOperandConstraintLacksEqualsCharacter,
  75   MatchingConstraintNotValidInOutputOperand,
  76
  77   // Input only
  78   InputOperandConstraintContainsEqualsCharacter,
  79   MatchingConstraintReferencesInvalidOperandNumber,
  80
  81   // Both
  82   PercentConstraintUsedWithLastOperand
  83 };
  84
  85 //===---------------------------------------------------------------------===//
  86
  87 Blocks should not capture variables that are only used in dead code.
  88
  89 The rule that we came up with is that blocks are required to capture
  90 variables if they're referenced in evaluated code, even if that code
  91 doesn't actually rely on the value of the captured variable.
  92
  93 For example, this requires a capture:
  94   (void) var;
  95 But this does not:
  96   if (false) puts(var);
  97
  98 Summary of <rdar://problem/9851835>: if we implement this, we should
  99 warn about non-POD variables that are referenced but not captured, but
 100 only if the non-reachability is not due to macro or template
 101 metaprogramming.
 102
 103 //===---------------------------------------------------------------------===//
 104
 105 We can still apply a modified version of the constructor/destructor
 106 delegation optimization in cases of virtual inheritance where:
 107   - there is no function-try-block,
 108   - the constructor signature is not variadic, and
 109   - the parameter variables can safely be copied and repassed
 110     to the base constructor because either
 111     - they have not had their addresses taken by the vbase initializers or
 112     - they were passed indirectly.
 113
 114 //===---------------------------------------------------------------------===//