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