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Clang implements the part of the ARM ABI saying that certain functions (e.g., constructors and destructors) return "this", but Apple's version of gcc and llvm-gcc did not. The libstdc++ dylib on iOS 5 was built with llvm-gcc, which means that clang cannot safely assume that code from the C++ runtime will correctly follow the ABI. It is also possible to run into this problem when linking with other libraries built with gcc or llvm-gcc. Even though there is no way to reliably detect that situation, it is most likely to come up when targeting older versions of iOS. Disabling the optimization for any code targeting iOS 5 solves the libstdc++ problem and has a reasonably good chance of fixing the issue for other older libraries as well. <rdar://problem/16377159> llvm-svn: 205272
IRgen optimization opportunities. //===---------------------------------------------------------------------===// The common pattern of -- short x; // or char, etc (x == 10) -- generates an zext/sext of x which can easily be avoided. //===---------------------------------------------------------------------===// Bitfields accesses can be shifted to simplify masking and sign extension. For example, if the bitfield width is 8 and it is appropriately aligned then is is a lot shorter to just load the char directly. //===---------------------------------------------------------------------===// It may be worth avoiding creation of alloca's for formal arguments for the common situation where the argument is never written to or has its address taken. The idea would be to begin generating code by using the argument directly and if its address is taken or it is stored to then generate the alloca and patch up the existing code. In theory, the same optimization could be a win for block local variables as long as the declaration dominates all statements in the block. NOTE: The main case we care about this for is for -O0 -g compile time performance, and in that scenario we will need to emit the alloca anyway currently to emit proper debug info. So this is blocked by being able to emit debug information which refers to an LLVM temporary, not an alloca. //===---------------------------------------------------------------------===// We should try and avoid generating basic blocks which only contain jumps. At -O0, this penalizes us all the way from IRgen (malloc & instruction overhead), all the way down through code generation and assembly time. On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just direct branches! //===---------------------------------------------------------------------===//