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I noticed another instance of the issue where references to aliases were being replaced with aliasees, this time in InstCombine. In the instance that I saw it turned out to be only a QoI issue (a symbol ended up being missing from the symbol table due to the last reference to the alias being removed, preventing HWASAN from symbolizing a global reference), but it could easily have manifested as incorrect behaviour. Since this is the third such issue encountered (previously: D65118, D65314) it seems to be time to address this common error/QoI issue once and for all and make the strip* family of functions not look through aliases. Includes a test for the specific issue that I saw, but no doubt there are other similar bugs fixed here. As with D65118 this has been tested to make sure that the optimization isn't load bearing. I built Clang, Chromium for Linux, Android and Windows as well as the test-suite and there were no size regressions. Differential Revision: https://reviews.llvm.org/D66606 llvm-svn: 369697
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! //===---------------------------------------------------------------------===//