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150
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1 IRgen optimization opportunities.
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2
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3 //===---------------------------------------------------------------------===//
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4
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5 The common pattern of
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6 --
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7 short x; // or char, etc
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8 (x == 10)
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9 --
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10 generates an zext/sext of x which can easily be avoided.
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11
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12 //===---------------------------------------------------------------------===//
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13
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14 Bitfields accesses can be shifted to simplify masking and sign
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15 extension. For example, if the bitfield width is 8 and it is
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16 appropriately aligned then is is a lot shorter to just load the char
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17 directly.
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18
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19 //===---------------------------------------------------------------------===//
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20
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21 It may be worth avoiding creation of alloca's for formal arguments
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22 for the common situation where the argument is never written to or has
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23 its address taken. The idea would be to begin generating code by using
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24 the argument directly and if its address is taken or it is stored to
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25 then generate the alloca and patch up the existing code.
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26
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27 In theory, the same optimization could be a win for block local
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28 variables as long as the declaration dominates all statements in the
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29 block.
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30
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31 NOTE: The main case we care about this for is for -O0 -g compile time
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32 performance, and in that scenario we will need to emit the alloca
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33 anyway currently to emit proper debug info. So this is blocked by
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34 being able to emit debug information which refers to an LLVM
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35 temporary, not an alloca.
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36
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37 //===---------------------------------------------------------------------===//
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38
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39 We should try and avoid generating basic blocks which only contain
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40 jumps. At -O0, this penalizes us all the way from IRgen (malloc &
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41 instruction overhead), all the way down through code generation and
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42 assembly time.
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43
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44 On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just
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45 direct branches!
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46
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47 //===---------------------------------------------------------------------===//
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