[vm, compiler] Add Location::MayBeSameAsInput.

`op x, x, y` is often shorter than `op z, x, y` in RISC-V because of the availablity of compressed instructions, but unconditionally using the two-address form requires extra moves when x is live after the instruction.

dart2js.aot.rv64 19767848 -> 19751424

TEST=ci
Change-Id: I92bf2a84fa91bb627e79853da94da51ee7924c34
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/305600
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/compiler/backend/flow_graph_compiler.cc

@@ -1834,6 +1834,7 @@ void FlowGraphCompiler::AllocateRegistersLocally(Instruction* instr) {
         break;
       case Location::kSameAsFirstInput:
       case Location::kSameAsFirstOrSecondInput:
+      case Location::kMayBeSameAsFirstInput:
         result_location = locs->in(0);
         break;
       case Location::kRequiresFpuRegister:

runtime/vm/compiler/backend/il_riscv.cc

@@ -3233,9 +3233,8 @@ LocationSummary* BinarySmiOpInstr::MakeLocationSummary(Zone* zone,
   if (kNumTemps == 1) {
     summary->set_temp(0, Location::RequiresRegister());
   }
-  // We make use of 3-operand instructions by not requiring result register
-  // to be identical to first input register as on Intel.
-  summary->set_out(0, Location::RequiresRegister());
+  summary->set_out(0, CanDeoptimize() ? Location::RequiresRegister()
+                                      : Location::MayBeSameAsFirstInput());
   return summary;
 }
 
@@ -4340,9 +4339,8 @@ LocationSummary* UnarySmiOpInstr::MakeLocationSummary(Zone* zone,
   LocationSummary* summary = new (zone)
       LocationSummary(zone, kNumInputs, kNumTemps, LocationSummary::kNoCall);
   summary->set_in(0, Location::RequiresRegister());
-  // We make use of 3-operand instructions by not requiring result register
-  // to be identical to first input register as on Intel.
-  summary->set_out(0, Location::RequiresRegister());
+  summary->set_out(0, CanDeoptimize() ? Location::RequiresRegister()
+                                      : Location::MayBeSameAsFirstInput());
   return summary;
 }
 
@@ -5556,7 +5554,7 @@ LocationSummary* BinaryInt64OpInstr::MakeLocationSummary(Zone* zone,
           zone, kNumInputs, kNumTemps, LocationSummary::kNoCall);
       summary->set_in(0, Location::RequiresRegister());
       summary->set_in(1, LocationRegisterOrConstant(right()));
-      summary->set_out(0, Location::RequiresRegister());
+      summary->set_out(0, Location::MayBeSameAsFirstInput());
       return summary;
     }
   }
@@ -6171,7 +6169,7 @@ LocationSummary* UnaryInt64OpInstr::MakeLocationSummary(Zone* zone,
   LocationSummary* summary = new (zone)
       LocationSummary(zone, kNumInputs, kNumTemps, LocationSummary::kNoCall);
   summary->set_in(0, Location::RequiresRegister());
-  summary->set_out(0, Location::RequiresRegister());
+  summary->set_out(0, Location::MayBeSameAsFirstInput());
   return summary;
 #endif
 }
@@ -6900,8 +6898,7 @@ void ConditionInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
             << kBoolValueBitPosition) == kTrueOffsetFromNull);
     __ SetIf(InvertCondition(true_condition), result);
     __ addi(result, result, kTrueOffsetFromNull >> kBoolValueBitPosition);
-    __ slli(result, result, kBoolValueBitPosition);
-    __ add(result, result, NULL_REG);
+    __ AddShifted(result, NULL_REG, result, kBoolValueBitPosition);
   }
 }
 

runtime/vm/compiler/backend/linearscan.cc

@@ -1487,6 +1487,16 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
     locs->set_out(0, Location::SameAsFirstInput());
   }
 
+  if (locs->out(0).IsUnallocated() &&
+      (locs->out(0).policy() == Location::kMayBeSameAsFirstInput)) {
+    auto input_defn = current->InputAt(0)->definition();
+    if (IsDeadAfterCurrentInstruction(block, current, input_defn)) {
+      locs->set_out(0, Location::SameAsFirstInput());
+    } else {
+      locs->set_out(0, Location::RequiresRegister());
+    }
+  }
+
   const bool output_same_as_first_input =
       locs->out(0).IsUnallocated() &&
       (locs->out(0).policy() == Location::kSameAsFirstInput);

runtime/vm/compiler/backend/locations.cc

@@ -409,6 +409,8 @@ const char* Location::Name() const {
           return "0";
         case kSameAsFirstOrSecondInput:
           return "0|1";
+        case kMayBeSameAsFirstInput:
+          return "0?";
         case kRequiresStack:
           return "RS";
       }

runtime/vm/compiler/backend/locations.h

@@ -337,6 +337,7 @@ class Location : public ValueObject {
     kWritableRegister,
     kSameAsFirstInput,
     kSameAsFirstOrSecondInput,
+    kMayBeSameAsFirstInput,
     // Forces the location to be spilled to the stack.
     // Currently only used for `Handle` arguments in `FfiCall` instructions.
     // Only available in optimized mode.
@@ -398,6 +399,12 @@ class Location : public ValueObject {
     return UnallocatedLocation(kSameAsFirstOrSecondInput);
   }
 
+  // Used for a three address instruction that can let its output be
+  // the same as an input if convenient.
+  static Location MayBeSameAsFirstInput() {
+    return UnallocatedLocation(kMayBeSameAsFirstInput);
+  }
+
   // Empty location. Used if there the location should be ignored.
   static Location NoLocation() { return Location(); }