[RISCV] Correct the limit of RegPresureSet GPRAll

The generated limit is 33, which is the total number of scalar registers
plus 1 (for `DUMMY_REG_PAIR_WITH_X0`).

This is not right as not all scalar registers can be used. There are 4-6
reserved registers, so we need to adjust the limit by the reserved set.

This change has impacts on instruction scheduling, MachineLICM, etc.

Here are the statistics of spills/reloads on `llvm-test-suite` with
`-O3 -march=rva23u64`:

```
Metric: regalloc.NumSpills,regalloc.NumReloads

Program                                       regalloc.NumSpills                  regalloc.NumReloads
                                              baseline           after    diff    baseline            after    diff
External/S...NT2017rate/502.gcc_r/502.gcc_r   11812.00           11338.00 -474.00 26813.00            25751.00 -1062.00
External/S...T2017speed/602.gcc_s/602.gcc_s   11812.00           11338.00 -474.00 26813.00            25751.00 -1062.00
External/S...te/526.blender_r/526.blender_r   13514.00           13228.00 -286.00 27456.00            27260.00  -196.00
External/S...00.perlbench_s/600.perlbench_s    4398.00            4274.00 -124.00  9745.00             9341.00  -404.00
External/S...00.perlbench_r/500.perlbench_r    4398.00            4274.00 -124.00  9745.00             9341.00  -404.00
SingleSour...nchmarks/Adobe-C++/loop_unroll    1533.00            1413.00 -120.00  2943.00             2633.00  -310.00
External/S...rate/510.parest_r/510.parest_r   43985.00           43879.00 -106.00 87409.00            87309.00  -100.00
External/S...te/538.imagick_r/538.imagick_r    4160.00            4060.00 -100.00 10338.00            10244.00   -94.00
External/S...ed/638.imagick_s/638.imagick_s    4160.00            4060.00 -100.00 10338.00            10244.00   -94.00
MultiSourc...e/Applications/ClamAV/clamscan    2120.00            2023.00  -97.00  5035.00             4901.00  -134.00
MultiSourc...sumer-typeset/consumer-typeset    1218.00            1129.00  -89.00  3041.00             2887.00  -154.00
MultiSourc.../Applications/JM/ldecod/ldecod    1341.00            1263.00  -78.00  2316.00             2238.00   -78.00
External/S...rate/511.povray_r/511.povray_r    1734.00            1659.00  -75.00  3413.00             3246.00  -167.00
MultiSource/Applications/SPASS/SPASS           1442.00            1376.00  -66.00  2954.00             2837.00  -117.00
MultiSourc.../DOE-ProxyApps-C++/CLAMR/CLAMR    1628.00            1568.00  -60.00  3026.00             2958.00   -68.00
      regalloc.NumSpills                            regalloc.NumReloads
run             baseline         after         diff            baseline         after         diff
mean   86.725206          85.041122    -1.684083     1363.122137         1342.900383  -3.212869
```

Co-authored-by: BoyaoWang430 <[email protected]>

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -934,3 +934,17 @@ bool RISCVRegisterInfo::getRegAllocationHints(
 
   return BaseImplRetVal;
 }
+
+unsigned RISCVRegisterInfo::getRegPressureSetLimit(const MachineFunction &MF,
+                                                   unsigned Idx) const {
+  if (Idx == RISCV::RegisterPressureSets::GPRAll) {
+    unsigned Reserved = 0;
+    BitVector ReservedRegs = getReservedRegs(MF);
+    for (MCPhysReg Reg = RISCV::X0_H; Reg <= RISCV::X31_H; Reg++)
+      if (ReservedRegs.test(Reg))
+        Reserved++;
+
+    return 32 - Reserved;
+  }
+  return RISCVGenRegisterInfo::getRegPressureSetLimit(MF, Idx);
+}

llvm/lib/Target/RISCV/RISCVRegisterInfo.h

@@ -144,6 +144,8 @@ struct RISCVRegisterInfo : public RISCVGenRegisterInfo {
   static bool isRVVRegClass(const TargetRegisterClass *RC) {
     return RISCVRI::isVRegClass(RC->TSFlags);
   }
+  unsigned getRegPressureSetLimit(const MachineFunction &MF,
+                                  unsigned Idx) const override;
 };
 } // namespace llvm