[RISCV] Allow compatible VTYPE in AVL Reg Forward cases

During insertion of VSETVLI, we have two related bits of code which decide whether we can reuse a previous vsetvli result. As was pointed out in the original review, these cases can allow any prior state for which we know that VL is the same for any value of AVL.

This was originally separated out of a desire for separate tests and review. As it turns out, finding a test case for this has been quite challenging. Most of the cases I tried, we manage to already get through other chains of logic. We do have one correct test change, but that only exercises one of the two changes.

Differential Revision: https://reviews.llvm.org/D126400

diff --git a/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp b/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
index 0ee92d3..afd8075 100644 (file)
--- a/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
@@ -202,6 +202,9 @@ public:
   }
 
   // Check if the VTYPE for these two VSETVLIInfos produce the same VLMAX.
+  // Note that having the same VLMAX ensures that both share the same
+  // function from AVL to VL; that is, they must produce the same VL value
+  // for any given AVL value.
   bool hasSameVLMAX(const VSETVLIInfo &Other) const {
     assert(isValid() && Other.isValid() &&
            "Can't compare invalid VSETVLIInfos");
@@ -717,7 +720,7 @@ bool RISCVInsertVSETVLI::needVSETVLI(const VSETVLIInfo &Require,
     return false;
 
   // We didn't find a compatible value. If our AVL is a virtual register,
-  // it might be defined by a VSET(I)VLI. If it has the same VTYPE we need
+  // it might be defined by a VSET(I)VLI. If it has the same VLMAX we need
   // and the last VL/VTYPE we observed is the same, we don't need a
   // VSETVLI here.
   if (!CurInfo.isUnknown() && Require.hasAVLReg() &&
@@ -726,7 +729,7 @@ bool RISCVInsertVSETVLI::needVSETVLI(const VSETVLIInfo &Require,
     if (MachineInstr *DefMI = MRI->getVRegDef(Require.getAVLReg())) {
       if (isVectorConfigInstr(*DefMI)) {
         VSETVLIInfo DefInfo = getInfoForVSETVLI(*DefMI);
-        if (DefInfo.hasSameAVL(CurInfo) && DefInfo.hasSameVTYPE(CurInfo))
+        if (DefInfo.hasSameAVL(CurInfo) && DefInfo.hasSameVLMAX(CurInfo))
           return false;
       }
     }
@@ -1237,7 +1240,7 @@ void RISCVInsertVSETVLI::doLocalPrepass(MachineBasicBlock &MBB) {
           }
         }
 
-        // If AVL is defined by a vsetvli with the same vtype, we can
+        // If AVL is defined by a vsetvli with the same VLMAX, we can
         // replace the AVL operand with the AVL of the defining vsetvli.
         // We avoid general register AVLs to avoid extending live ranges
         // without being sure we can kill the original source reg entirely.
@@ -1246,7 +1249,7 @@ void RISCVInsertVSETVLI::doLocalPrepass(MachineBasicBlock &MBB) {
           if (MachineInstr *DefMI = MRI->getVRegDef(Require.getAVLReg())) {
             if (isVectorConfigInstr(*DefMI)) {
               VSETVLIInfo DefInfo = getInfoForVSETVLI(*DefMI);
-              if (DefInfo.hasSameVTYPE(Require) &&
+              if (DefInfo.hasSameVLMAX(Require) &&
                   (DefInfo.hasAVLImm() || DefInfo.getAVLReg() == RISCV::X0)) {
                 MachineOperand &VLOp = MI.getOperand(getVLOpNum(MI));
                 if (DefInfo.hasAVLImm())

diff --git a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
index 9b92926..b554b11 100644 (file)
--- a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
@@ -816,7 +816,6 @@ define <vscale x 4 x i32> @cross_block_mutate(<vscale x 4 x i32> %a, <vscale x 4
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    vsetivli a0, 6, e32, m2, tu, mu
 ; CHECK-NEXT:    vmv.s.x v8, a0
-; CHECK-NEXT:    vsetvli zero, a0, e32, m2, tu, mu
 ; CHECK-NEXT:    vadd.vv v8, v8, v10, v0.t
 ; CHECK-NEXT:    ret
                                          <vscale x 4 x i1> %mask) {