Correct the vqrdmlah intrinsics. #1246
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These intrinsics were added in 8.1 and should emit a sqrdmlah, however they currently emit sqrdmulh followed by a sqadd. LLVM doesn't convert this into sqrdmlah without enabling the rdm feature.
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r? @Amanieu (rust-highfive has picked a reviewer for you, use r? to override) |
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By the way, could you send a bug report to LLVM about the incorrect optimization? |
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Yes, I was going to raise it, I'm just waiting on an account on the bug tracker. |
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The
sqrdmulhfollowed by asqaddis only optimized intosqrdmlahwhen the rdm target feature is enabled. I believe this is actually an incorrect optimization in LLVM though. However this patch will cause the correct instruction for the intrinsic, and I couldn't see another way to generate this instruction.For instance using
a = 0x80000000, b = 0x80000000, c = 0x80000000sqrdmlahwould be0whereas thesqrdmulhsqaddsequence would be-1