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LLVM backend for 6502
e6329cf303
found by a single test reduced out of a failure on llvm-stress. The start of the problem (and the crash) came when we tried to use a find of a non-used slot in the move-to half of the move-mask as the target for two bad-half inputs. While if lucky this will be the first of a pair of slots which we can place the bad-half inputs into, it isn't actually guaranteed. This really isn't surprising, not sure what I was thinking. The correct way to find the two unused slots is to look for one of the *used* slots. We know it isn't that pair, and we can use some modular arithmetic to find the other pair by masking off the odd bit and adding 2 modulo 4. With this, we reliably found a viable pair of slots for the bad-half inputs. Sadly, that wasn't enough. We also had a wrong code bug that surfaced when I reduced the test case for this where we would use the same slot twice for the two bad inputs. This is because both of the bad inputs could be in odd slots originally and thus the mod-2 mapping would actually be the same. The whole point of the weird indexing into the pair of empty slots was to try to leverage when the end result needed the two bad-half inputs to be paired in a dword and pre-pair them in the correct orrientation. This is less important with the powerful combining we're now doing, and also easier and more reliable to achieve be noting that we add the bad-half inputs in order. Thus, if they are in a dword pair, the low part of that will be the first input in the sequence. Always putting that in the low element will just do the right thing in addition to computing the correct result. Test case added. =] git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214849 91177308-0d34-0410-b5e6-96231b3b80d8 |
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| README.txt | ||
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