Specialize mint shift code on arm for a constant shift amount.

runtime/vm/intermediate_language_ia32.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_IA32.
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 6020 matching lines @@
             }
           } else {
             __ shld(left_hi, left_lo, Immediate(shift));
             __ shll(left_lo, Immediate(shift));
           }
         }
         break;
       }
       default:
         UNREACHABLE();
-        break;
     }
   } else {
     // Code for a variable shift amount.
     // Deoptimize if shift count is > 63.
     // sarl operation masks the count to 5 bits and
     // shrd is undefined with count > operand size (32)
     __ SmiUntag(ECX);
     if (has_shift_count_check()) {
       __ cmpl(ECX, Immediate(kMintShiftCountLimit));
       __ j(ABOVE, deopt);
     }
     Label done, large_shift;
     switch (op_kind()) {
       case Token::kSHR: {
         __ cmpl(ECX, Immediate(31));
         __ j(ABOVE, &large_shift);
 
         __ shrd(left_lo, left_hi);  // Shift count in CL.
         __ sarl(left_hi, ECX);  // Shift count in CL.
         __ jmp(&done, Assembler::kNearJump);
 
         __ Bind(&large_shift);
-        __ subl(ECX, Immediate(32));
+        // No need to subtract 32 from CL, only 5 bits used by sarl.
         __ movl(left_lo, left_hi);  // Shift by 32.
         __ sarl(left_hi, Immediate(31));  // Sign extend left hi.
-        __ sarl(left_lo, ECX);  // Shift count - 32 in CL.
+        __ sarl(left_lo, ECX);  // Shift count: CL % 32.
         break;
       }
       case Token::kSHL: {
         if (can_overflow()) {
           Register temp1 = locs()->temp(0).reg();
           Register temp2 = locs()->temp(1).reg();
           __ movl(temp1, left_hi);  // Preserve high 32 bits.
           __ cmpl(ECX, Immediate(31));
           __ j(ABOVE, &large_shift);
 
           __ shld(left_hi, left_lo);  // Shift count in CL.
           __ shll(left_lo, ECX);  // Shift count in CL.
           // Check for overflow by shifting back the high 32 bits
           // and comparing with the input.
           __ movl(temp2, left_hi);
           __ sarl(temp2, ECX);
           __ cmpl(temp1, temp2);
           __ j(NOT_EQUAL, deopt);
           __ jmp(&done, Assembler::kNearJump);
 
           __ Bind(&large_shift);
-          __ subl(ECX, Immediate(32));
+          // No need to subtract 32 from CL, only 5 bits used by shll.
           __ movl(left_hi, left_lo);  // Shift by 32.
           __ xorl(left_lo, left_lo);  // Zero left_lo.
-          __ shll(left_hi, ECX);  // Shift count in CL.
+          __ shll(left_hi, ECX);  // Shift count: CL % 32.
           // Check for overflow by sign extending the high 32 bits
           // and comparing with the input.
           __ movl(temp2, left_hi);
           __ sarl(temp2, Immediate(31));
           __ cmpl(temp1, temp2);
           __ j(NOT_EQUAL, deopt);
         } else {
           __ cmpl(ECX, Immediate(31));
           __ j(ABOVE, &large_shift);
 
           __ shld(left_hi, left_lo);  // Shift count in CL.
           __ shll(left_lo, ECX);  // Shift count in CL.
           __ jmp(&done, Assembler::kNearJump);
 
           __ Bind(&large_shift);
-          __ subl(ECX, Immediate(32));
+          // No need to subtract 32 from CL, only 5 bits used by shll.
           __ movl(left_hi, left_lo);  // Shift by 32.
           __ xorl(left_lo, left_lo);  // Zero left_lo.
-          __ shll(left_hi, ECX);  // Shift count in CL.
+          __ shll(left_hi, ECX);  // Shift count: CL % 32.
         }
         break;
       }
       default:
         UNREACHABLE();
         break;
     }
     __ Bind(&done);
   }
   if (FLAG_throw_on_javascript_int_overflow) {
@@ skipping 709 matching lines @@
   __ movl(EDX, Immediate(kInvalidObjectPointer));
   __ movl(EDX, Immediate(kInvalidObjectPointer));
 #endif
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_IA32