[RISCV] Use LiveIntervals to determine if AVL dominates when coalescing (llvm#118285)

In order to coalesce a vsetvli with a register AVL into a previous
vsetvli, we need to make sure that the AVL register is reachable at the
previous vsetvli.

Back in pre-RA vsetvli insertion we just checked to see if the two
virtual registers were the same virtual register, and then this was
hacked around in the move to post-RA. We can instead use live intervals
to check that the reaching definition is the same at both instructions.

On its own this doesn't have much of an impact, but helps a lot in
llvm#118283 and enables coalescing in about 60 of the test cases from that
PR.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -1619,14 +1619,15 @@ bool RISCVInsertVSETVLI::canMutatePriorConfig(
     }
 
     auto &AVL = MI.getOperand(1);
-    auto &PrevAVL = PrevMI.getOperand(1);
 
-    // If the AVL is a register, we need to make sure MI's AVL dominates PrevMI.
-    // For now just check that PrevMI uses the same virtual register.
-    if (AVL.isReg() && AVL.getReg() != RISCV::X0 &&
-        (!MRI->hasOneDef(AVL.getReg()) || !PrevAVL.isReg() ||
-         PrevAVL.getReg() != AVL.getReg()))
-      return false;
+    // If the AVL is a register, we need to make sure its definition is the same
+    // at PrevMI as it was at MI.
+    if (AVL.isReg() && AVL.getReg() != RISCV::X0) {
+      VNInfo *VNI = getVNInfoFromReg(AVL.getReg(), MI, LIS);
+      VNInfo *PrevVNI = getVNInfoFromReg(AVL.getReg(), PrevMI, LIS);
+      if (!VNI || !PrevVNI || VNI != PrevVNI)
+        return false;
+    }
   }
 
   assert(PrevMI.getOperand(2).isImm() && MI.getOperand(2).isImm());

llvm/test/CodeGen/RISCV/rvv/compressstore.ll

@@ -652,15 +652,14 @@ define void @test_compresstore_v64i32(ptr %p, <64 x i1> %mask, <64 x i32> %data)
 ; RV64-NEXT:    vse32.v v24, (a0)
 ; RV64-NEXT:    vsetivli zero, 4, e8, mf2, ta, ma
 ; RV64-NEXT:    vslidedown.vi v8, v0, 4
-; RV64-NEXT:    vsetvli zero, zero, e32, m2, ta, ma
-; RV64-NEXT:    vmv.x.s a2, v0
 ; RV64-NEXT:    vsetvli zero, a1, e32, m8, ta, ma
+; RV64-NEXT:    vmv.x.s a1, v0
 ; RV64-NEXT:    vcompress.vm v24, v16, v8
-; RV64-NEXT:    vcpop.m a1, v8
-; RV64-NEXT:    cpopw a2, a2
-; RV64-NEXT:    slli a2, a2, 2
-; RV64-NEXT:    add a0, a0, a2
-; RV64-NEXT:    vsetvli zero, a1, e32, m8, ta, ma
+; RV64-NEXT:    vcpop.m a2, v8
+; RV64-NEXT:    cpopw a1, a1
+; RV64-NEXT:    slli a1, a1, 2
+; RV64-NEXT:    add a0, a0, a1
+; RV64-NEXT:    vsetvli zero, a2, e32, m8, ta, ma
 ; RV64-NEXT:    vse32.v v24, (a0)
 ; RV64-NEXT:    ret
 ;