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8a2f3c778e
The X86-64 ABI code didn't handle the case when a struct would get classified and turn up as "NoClass INTEGER" for example. This is perfectly possible when the first slot is all padding (e.g. due to empty base classes). In this situation, the first 8-byte doesn't take a register at all, only the second 8-byte does. This fixes this by enhancing the x86-64 abi stuff to allow and handle this case, reverts the broken fix for PR5831, and enhances the target independent stuff to be able to handle an argument value in registers being accessed at an offset from the memory value. This is the last x86-64 calling convention related miscompile that I'm aware of. llvm-svn: 109848
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! //===---------------------------------------------------------------------===//