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Much to my surprise, '-disable-llvm-optzns' which I thought was the magical flag I wanted to get at the raw LLVM IR coming out of Clang deosn't do that. It still runs some passes over the IR. I don't want that, I really want the *raw* IR coming out of Clang and I strongly suspect everyone else using it is in the same camp. There is actually a flag that does what I want that I didn't know about called '-disable-llvm-passes'. I suspect many others don't know about it either. It both does what I want and is much simpler. This removes the confusing version and makes that spelling of the flag an alias for '-disable-llvm-passes'. I've also moved everything in Clang to use the 'passes' spelling as it seems both more accurate (*all* LLVM passes are disabled, not just optimizations) and much easier to remember and spell correctly. This is part of simplifying how Clang drives LLVM to make it cleaner to wire up to the new pass manager. Differential Revision: https://reviews.llvm.org/D28047 llvm-svn: 290392
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! //===---------------------------------------------------------------------===//