Do not emit undefined lshr/ashr for Neon shifts #1238
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r? @Amanieu (rust-highfive has picked a reviewer for you, use r? to override) |
In LLVM the lshr and ashr instruction is undefined when the amount is equal to the element size, but Neon shifts support such shifts. With signed values, when shifting by the element size is requested subtract one as right shifting a signed value by its size is equivalent to a shift of size - 1. When shifting an unsigned value by the element size, return a vector of all 0's as that's what the shift would result in.
JamieCunliffe
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Should the constraint on the immediate be changed to exclude 64? Is 64 a valid argument for these immediates? |
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64 is valid for these, https://developer.arm.com/architectures/instruction-sets/intrinsics/vshrd_n_s64 |
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In LLVM the lshr and ashr instruction is undefined when the amount is
equal to the element size, but Neon shifts support such shifts.
With signed values, when shifting by the element size is requested
subtract one as right shifting a signed value by its size is
equivalent to a shift of size - 1.
When shifting an unsigned value by the element size, return a vector
of all 0's as that's what the shift would result in.