[llvm] [RISCV] Minimally modify incoming state in transferBefore (PR #72352)

[Luke Lau via llvm-commits]

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@@ -1013,73 +1015,76 @@ bool RISCVInsertVSETVLI::needVSETVLI(const MachineInstr &MI,
   return true;
 }
 
-// Given an incoming state reaching MI, modifies that state so that it is minimally
-// compatible with MI.  The resulting state is guaranteed to be semantically legal
-// for MI, but may not be the state requested by MI.
+// Given an incoming state reaching MI, minimally modifies that state so that it
+// is compatible with MI. The resulting state is guaranteed to be semantically
+// legal for MI, but may not be the state requested by MI.
 void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
                                         const MachineInstr &MI) const {
   uint64_t TSFlags = MI.getDesc().TSFlags;
   if (!RISCVII::hasSEWOp(TSFlags))
     return;
 
-  const VSETVLIInfo NewInfo = computeInfoForInstr(MI, TSFlags, MRI);
+  VSETVLIInfo NewInfo = computeInfoForInstr(MI, TSFlags, MRI);
   if (Info.isValid() && !needVSETVLI(MI, NewInfo, Info))
     return;
 
-  const VSETVLIInfo PrevInfo = Info;
-  Info = NewInfo;
+  if (Info.hasSEWLMULRatioOnly() || !Info.isValid() || Info.isUnknown())
+    Info = NewInfo;
 
-  if (!RISCVII::hasVLOp(TSFlags))
-    return;
+  DemandedFields Demanded = getDemanded(MI, MRI, ST);
 
   // If we don't use LMUL or the SEW/LMUL ratio, then adjust LMUL so that we
   // maintain the SEW/LMUL ratio. This allows us to eliminate VL toggles in more
   // places.
-  DemandedFields Demanded = getDemanded(MI, MRI, ST);
   if (!Demanded.LMUL && !Demanded.SEWLMULRatio && Info.isValid() &&
-      PrevInfo.isValid() && !Info.isUnknown() && !PrevInfo.isUnknown()) {
-    if (auto NewVLMul = RISCVVType::getSameRatioLMUL(
-            PrevInfo.getSEW(), PrevInfo.getVLMUL(), Info.getSEW()))
-      Info.setVLMul(*NewVLMul);
-  }
-
-  // For vmv.s.x and vfmv.s.f, there are only two behaviors, VL = 0 and
-  // VL > 0. We can discard the user requested AVL and just use the last
-  // one if we can prove it equally zero.  This removes a vsetvli entirely
-  // if the types match or allows use of cheaper avl preserving variant
-  // if VLMAX doesn't change.  If VLMAX might change, we couldn't use
-  // the 'vsetvli x0, x0, vtype" variant, so we avoid the transform to
-  // prevent extending live range of an avl register operand.
-  // TODO: We can probably relax this for immediates.
-  if (isScalarInsertInstr(MI) && PrevInfo.isValid() &&
----------------
lukel97 wrote:

Done in 69f64dedb09bf39bd3ad66bcbc2c947b23342a11

https://github.com/llvm/llvm-project/pull/72352