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ca323cf916
much more effectively when trying to constant fold a load of a constant. Previously, we only handled bitcasts by trying to find a totally generic byte representation of the constant and use that. Now, we look through the bitcast to see what constant we might fold the load into, and then try to form a constant expression cast of the found value that would be equivalent to loading the value. You might wonder why on earth this actually matters. Well, turns out that the Itanium ABI causes us to create a single array for a vtable where the first elements are virtual base offsets, followed by the virtual function pointers. Because the array is homogenous the element type is consistently i8* and we inttoptr the virtual base offsets into the initial elements. Then constructors bitcast these pointers to i64 pointers prior to loading them. Boom, no more constant folding of virtual base offsets. This is the first fix to LLVM to address the *insane* performance Eric Niebler discovered with Clang on his range comprehensions[1]. There is more to come though, this doesn't *really* fix the problem fully. [1]: http://ericniebler.com/2014/04/27/range-comprehensions/ git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208856 91177308-0d34-0410-b5e6-96231b3b80d8 |
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