Fix mint shifting by zero on MIPS (issue 23117).

runtime/vm/intermediate_language_mips.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 4981 matching lines @@
         }
         break;
       }
       default:
         UNREACHABLE();
     }
   } else {
     // Code for a variable shift amount.
     Register shift = locs()->in(1).reg();
 
+    // Code below assumes shift amount is not 0 (cannot shift by 32 - 0).
+    Label non_zero_shift, done;
+    __ bne(shift, ZR, &non_zero_shift);
+    __ delay_slot()->mov(out_lo, left_lo);
+    __ b(&done);
+    __ delay_slot()->mov(out_hi, left_hi);
+    __ Bind(&non_zero_shift);
+
     // Deopt if shift is larger than 63 or less than 0.
     if (has_shift_count_check()) {
       __ sltiu(CMPRES1, shift, Immediate(2*(kMintShiftCountLimit + 1)));
       __ beq(CMPRES1, ZR, deopt);
       // Untag shift count.
       __ delay_slot()->SmiUntag(shift);
     } else {
       // Untag shift count.
       __ SmiUntag(shift);
     }
 
     switch (op_kind()) {
       case Token::kSHR: {
-        Label large_shift, done;
+        Label large_shift;
         __ sltiu(CMPRES1, shift, Immediate(32));
         __ beq(CMPRES1, ZR, &large_shift);
 
-        // shift < 32.
+        // 0 < shift < 32.
         __ delay_slot()->ori(TMP, ZR, Immediate(32));
         __ subu(TMP, TMP, shift);  // TMP = 32 - shift; 0 < TMP <= 31.
         __ sllv(out_lo, left_hi, TMP);
         __ srlv(TMP, left_lo, shift);
         __ or_(out_lo, out_lo, TMP);
         __ b(&done);
         __ delay_slot()->srav(out_hi, left_hi, shift);
 
         // shift >= 32.
         __ Bind(&large_shift);
         __ sra(out_hi, left_hi, 31);
         __ srav(out_lo, left_hi, shift);  // Only 5 low bits of shift used.
 
-        __ Bind(&done);
         break;
       }
       case Token::kSHL: {
-        Label large_shift, done;
+        Label large_shift;
         __ sltiu(CMPRES1, shift, Immediate(32));
         __ beq(CMPRES1, ZR, &large_shift);
 
-        // shift < 32.
+        // 0 < shift < 32.
         __ delay_slot()->ori(TMP, ZR, Immediate(32));
         __ subu(TMP, TMP, shift);  // TMP = 32 - shift; 0 < TMP <= 31.
         __ srlv(out_hi, left_lo, TMP);
         __ sllv(TMP, left_hi, shift);
         __ or_(out_hi, out_hi, TMP);
         // Check for overflow.
         if (can_overflow()) {
           // Compare high word from input with shifted high word from output.
           __ srav(TMP, out_hi, shift);
           __ beq(TMP, left_hi, &done);
@@ skipping 13 matching lines @@
           // high word from input to sign of output.
           // Overflow if they aren't equal.
           __ srav(TMP, out_hi, shift);
           __ bne(TMP, left_lo, deopt);
           __ delay_slot()->sra(TMP, out_hi, 31);
           __ bne(TMP, left_hi, deopt);
           __ delay_slot()->mov(out_lo, ZR);
         } else {
           __ mov(out_lo, ZR);
         }
-        __ Bind(&done);
         break;
       }
       default:
         UNREACHABLE();
     }
+    __ Bind(&done);
   }
 }
 
 
 LocationSummary* UnaryMintOpInstr::MakeLocationSummary(Zone* zone,
                                                        bool opt) const {
   const intptr_t kNumInputs = 1;
   const intptr_t kNumTemps = 0;
   LocationSummary* summary = new(zone) LocationSummary(
       zone, kNumInputs, kNumTemps, LocationSummary::kNoCall);
@@ skipping 518 matching lines @@
                                 1,
                                 locs());
   __ lw(result, Address(SP, 1 * kWordSize));
   __ addiu(SP, SP, Immediate(2 * kWordSize));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS