[clang-tools-extra] [clang] [libcxx] [compiler-rt] [lldb] [llvm] [flang] [lld] [libc] [TTI][RISCV]Improve costs for fixed vector whole reg extract/insert. (PR #80164)

[Philip Reames via cfe-commits]

================
@@ -326,6 +326,50 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
     switch (Kind) {
     default:
       break;
+    case TTI::SK_ExtractSubvector:
+      if (isa<FixedVectorType>(SubTp)) {
+        unsigned TpRegs = getRegUsageForType(Tp);
+        unsigned NumElems =
+            divideCeil(Tp->getElementCount().getFixedValue(), TpRegs);
+        // Whole vector extract - just the vector itself + (possible) vsetvli.
+        // TODO: consider adding the cost for vsetvli.
+        if (Index == 0 || (ST->getRealMaxVLen() == ST->getRealMinVLen() &&
----------------
preames wrote:

I think this check would be more clearly expressed as an and of the following clauses
a) ST->getRealMaxVLen() == ST->getRealMinVLen()
b) NumElems * ElementSizeInBits == VLEN
c) Index % NumElems == 0

Note that this only supports m1 full extracts.  But starting there and extending it to m2, and m4 later seems entirely reasonable.

https://github.com/llvm/llvm-project/pull/80164