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      1 IRgen optimization opportunities.
      2 
      3 //===---------------------------------------------------------------------===//
      4 
      5 The common pattern of
      6 --
      7 short x; // or char, etc
      8 (x == 10)
      9 --
     10 generates an zext/sext of x which can easily be avoided.
     11 
     12 //===---------------------------------------------------------------------===//
     13 
     14 Bitfields accesses can be shifted to simplify masking and sign
     15 extension. For example, if the bitfield width is 8 and it is
     16 appropriately aligned then is is a lot shorter to just load the char
     17 directly.
     18 
     19 //===---------------------------------------------------------------------===//
     20 
     21 It may be worth avoiding creation of alloca's for formal arguments
     22 for the common situation where the argument is never written to or has
     23 its address taken. The idea would be to begin generating code by using
     24 the argument directly and if its address is taken or it is stored to
     25 then generate the alloca and patch up the existing code.
     26 
     27 In theory, the same optimization could be a win for block local
     28 variables as long as the declaration dominates all statements in the
     29 block.
     30 
     31 NOTE: The main case we care about this for is for -O0 -g compile time
     32 performance, and in that scenario we will need to emit the alloca
     33 anyway currently to emit proper debug info. So this is blocked by
     34 being able to emit debug information which refers to an LLVM
     35 temporary, not an alloca.
     36 
     37 //===---------------------------------------------------------------------===//
     38 
     39 We should try and avoid generating basic blocks which only contain
     40 jumps. At -O0, this penalizes us all the way from IRgen (malloc &
     41 instruction overhead), all the way down through code generation and
     42 assembly time.
     43 
     44 On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just
     45 direct branches!
     46 
     47 //===---------------------------------------------------------------------===//
     48