[llvm] [GlobalISel][RISCV] Use constant pool for large integer constants. (PR #81101)

[Matt Arsenault via llvm-commits]

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@@ -451,17 +460,94 @@ bool RISCVLegalizerInfo::legalizeVAStart(MachineInstr &MI,
   return true;
 }
 
+bool RISCVLegalizerInfo::shouldBeInConstantPool(APInt APImm,
+                                                bool ShouldOptForSize) const {
+  unsigned BitWidth = APImm.getBitWidth();
+  assert(BitWidth == 32 || BitWidth == 64);
+  int64_t Imm = APImm.getSExtValue();
+  // All simm32 constants should be handled by isel.
+  // NOTE: The getMaxBuildIntsCost call below should return a value >= 2 making
+  // this check redundant, but small immediates are common so this check
+  // should have better compile time.
+  if (isInt<32>(Imm))
+    return false;
+
+  // We only need to cost the immediate, if constant pool lowering is enabled.
+  if (!STI.useConstantPoolForLargeInts())
+    return false;
+
+  RISCVMatInt::InstSeq Seq = RISCVMatInt::generateInstSeq(Imm, STI);
+  if (Seq.size() <= STI.getMaxBuildIntsCost())
+    return false;
+
+  // Optimizations below are disabled for opt size. If we're optimizing for
+  // size, use a constant pool.
+  if (ShouldOptForSize)
+    return true;
+  //
+  // Special case. See if we can build the constant as (ADD (SLLI X, C), X) do
+  // that if it will avoid a constant pool.
+  // It will require an extra temporary register though.
+  // If we have Zba we can use (ADD_UW X, (SLLI X, 32)) to handle cases where
+  // low and high 32 bits are the same and bit 31 and 63 are set.
+  unsigned ShiftAmt, AddOpc;
+  RISCVMatInt::InstSeq SeqLo =
+      RISCVMatInt::generateTwoRegInstSeq(Imm, STI, ShiftAmt, AddOpc);
+  return !(!SeqLo.empty() && (SeqLo.size() + 2) <= STI.getMaxBuildIntsCost());
+}
+
+// TODO: This is almost the same as LegalizerHelper::lowerFConstant and is
+// target-independent. Should we move this to LegalizeHelper?
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arsenm wrote:

Yes, I'm surprised this isn't already implemented there 

https://github.com/llvm/llvm-project/pull/81101