S390: Used RISBG in MacroAssembler::IndexToArrayOffset to improve performance.

src/s390/macro-assembler-s390.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_S390_MACRO_ASSEMBLER_S390_H_
 #define V8_S390_MACRO_ASSEMBLER_S390_H_
 
 #include "src/assembler.h"
 #include "src/bailout-reason.h"
 #include "src/frames.h"
@@ skipping 1564 matching lines @@
     if (kSmiShift < elementSizeLog2) {
       ShiftLeftP(dst, src, Operand(elementSizeLog2 - kSmiShift));
     } else if (kSmiShift > elementSizeLog2) {
       ShiftRightArithP(dst, src, Operand(kSmiShift - elementSizeLog2));
     } else if (!dst.is(src)) {
       LoadRR(dst, src);
     }
   }
 
   void IndexToArrayOffset(Register dst, Register src, int elementSizeLog2,
-                          bool isSmi) {
+                          bool isSmi, bool keyMaybeNegative) {
     if (isSmi) {
       SmiToArrayOffset(dst, src, elementSizeLog2);
-    } else {
+    } else if (keyMaybeNegative ||
+          !CpuFeatures::IsSupported(GENERAL_INSTR_EXT)) {
 #if V8_TARGET_ARCH_S390X
+      // If array access is dehoisted, the key, being an int32, can contain
+      // a negative value, as needs to be sign-extended to 64-bit for
+      // memory access.
+      //
       // src (key) is a 32-bit integer.  Sign extension ensures
       // upper 32-bit does not contain garbage before being used to
       // reference memory.
       lgfr(src, src);
 #endif
       ShiftLeftP(dst, src, Operand(elementSizeLog2));
+    } else {
+      // Small optimization to reduce pathlength.  After Bounds Check,
+      // the key is guaranteed to be non-negative.  Leverage RISBG,
+      // which also performs zero-extension.
+      risbg(dst, src, Operand(32 - elementSizeLog2),
+            Operand(63 - elementSizeLog2), Operand(elementSizeLog2),
+            true);
     }
   }
 
   // Untag the source value into destination and jump if source is a smi.
   // Souce and destination can be the same register.
   void UntagAndJumpIfSmi(Register dst, Register src, Label* smi_case);
 
   // Untag the source value into destination and jump if source is not a smi.
   // Souce and destination can be the same register.
   void UntagAndJumpIfNotSmi(Register dst, Register src, Label* non_smi_case);
@@ skipping 291 matching lines @@
 #define ACCESS_MASM(masm)    \
   masm->stop(__FILE_LINE__); \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_S390_MACRO_ASSEMBLER_S390_H_