Improve code generated for AllocInNewSpace

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
===================================================================
--- src/arm/macro-assembler-arm.cc	(revision 5914)
+++ src/arm/macro-assembler-arm.cc	(working copy)
@@ -920,6 +920,7 @@
   }
 
   ASSERT(!result.is(scratch1));
+  ASSERT(!result.is(scratch2));
   ASSERT(!scratch1.is(scratch2));
 
   // Make object size into bytes.
@@ -928,38 +929,55 @@
   }
   ASSERT_EQ(0, object_size & kObjectAlignmentMask);
 
-  // Load address of new object into result and allocation top address into
-  // scratch1.
+  // Check relative positions of allocation top and limit addresses.
+  // The values must be adjacent in memory to allow the use of LDM.
+  // Also, assert that the registers are numbered such that the values
+  // are loaded in the correct order.
   ExternalReference new_space_allocation_top =
       ExternalReference::new_space_allocation_top_address();
-  mov(scratch1, Operand(new_space_allocation_top));
+  ExternalReference new_space_allocation_limit =
+      ExternalReference::new_space_allocation_limit_address();
+  uint32_t top   =
+      reinterpret_cast<uint32_t>(new_space_allocation_top.address());
+  uint32_t limit =
+      reinterpret_cast<uint32_t>(new_space_allocation_limit.address());
+  ASSERT((limit - top) == 4);

[Erik Corry, 2010/12/06 09:40:06]

Prefer kPointerSize to 4 since it tells the reader _why_ we have 4 here.

+  ASSERT(result.code() < ip.code());
+
+  // Set up allocation top address and object size registers.
+  Register topaddr = scratch1;
+  Register obj_size_reg = scratch2;
+  mov(topaddr, Operand(new_space_allocation_top));
+  mov(obj_size_reg, Operand(object_size));
+
+  // This code stores a temporary value in ip. This is OK, as the code below
+  // does not need ip for implicit literal generation.
   if ((flags & RESULT_CONTAINS_TOP) == 0) {
-    ldr(result, MemOperand(scratch1));
-  } else if (FLAG_debug_code) {
-    // Assert that result actually contains top on entry. scratch2 is used
-    // immediately below so this use of scratch2 does not cause difference with
-    // respect to register content between debug and release mode.
-    ldr(scratch2, MemOperand(scratch1));
-    cmp(result, scratch2);
-    Check(eq, "Unexpected allocation top");
+    // Load allocation top into result and allocation limit into ip.
+    ldm(ia, topaddr, result.bit() | ip.bit());
+  } else {
+    if (FLAG_debug_code) {
+      // Assert that result actually contains top on entry. ip is used
+      // immediately below so this use of ip does not cause difference with
+      // respect to register content between debug and release mode.
+      ldr(ip, MemOperand(topaddr));
+      cmp(result, ip);
+      Check(eq, "Unexpected allocation top");
+    }
+    // Load allocation limit into ip. Result already contains allocation top.
+    ldr(ip, MemOperand(topaddr, limit - top));
   }
 
   // Calculate new top and bail out if new space is exhausted. Use result
   // to calculate the new top.
-  ExternalReference new_space_allocation_limit =
-      ExternalReference::new_space_allocation_limit_address();
-  mov(scratch2, Operand(new_space_allocation_limit));
-  ldr(scratch2, MemOperand(scratch2));
-  add(result, result, Operand(object_size));
-  cmp(result, Operand(scratch2));
+  add(scratch2, result, Operand(obj_size_reg));
+  cmp(scratch2, Operand(ip));
   b(hi, gc_required);
-  str(result, MemOperand(scratch1));
+  str(scratch2, MemOperand(topaddr));
 
-  // Tag and adjust back to start of new object.
+  // Tag object if requested.
   if ((flags & TAG_OBJECT) != 0) {
-    sub(result, result, Operand(object_size - kHeapObjectTag));
-  } else {
-    sub(result, result, Operand(object_size));
+    add(result, result, Operand(kHeapObjectTag));
   }
 }
 
@@ -982,53 +1000,64 @@
   }
 
   ASSERT(!result.is(scratch1));
+  ASSERT(!result.is(scratch2));
   ASSERT(!scratch1.is(scratch2));
 
-  // Load address of new object into result and allocation top address into
-  // scratch1.
+  // Check relative positions of allocation top and limit addresses.
+  // The values must be adjacent in memory to allow the use of LDM.
+  // Also, assert that the registers are numbered such that the values
+  // are loaded in the correct order.
   ExternalReference new_space_allocation_top =
       ExternalReference::new_space_allocation_top_address();
-  mov(scratch1, Operand(new_space_allocation_top));
+  ExternalReference new_space_allocation_limit =
+      ExternalReference::new_space_allocation_limit_address();
+  uint32_t top =
+      reinterpret_cast<uint32_t>(new_space_allocation_top.address());
+  uint32_t limit =
+      reinterpret_cast<uint32_t>(new_space_allocation_limit.address());
+  ASSERT((limit - top) == 4);
+  ASSERT(result.code() < ip.code());
+
+  // Set up allocation top address.
+  Register topaddr = scratch1;
+  mov(topaddr, Operand(new_space_allocation_top));
+
+  // This code stores a temporary value in ip. This is OK, as the code below
+  // does not need ip for implicit literal generation.
   if ((flags & RESULT_CONTAINS_TOP) == 0) {
-    ldr(result, MemOperand(scratch1));
-  } else if (FLAG_debug_code) {
-    // Assert that result actually contains top on entry. scratch2 is used
-    // immediately below so this use of scratch2 does not cause difference with
-    // respect to register content between debug and release mode.
-    ldr(scratch2, MemOperand(scratch1));
-    cmp(result, scratch2);
-    Check(eq, "Unexpected allocation top");
+    // Load allocation top into result and allocation limit into ip.
+    ldm(ia, topaddr, result.bit() | ip.bit());
+  } else {
+    if (FLAG_debug_code) {
+      // Assert that result actually contains top on entry. ip is used
+      // immediately below so this use of ip does not cause difference with
+      // respect to register content between debug and release mode.
+      ldr(ip, MemOperand(topaddr));
+      cmp(result, ip);
+      Check(eq, "Unexpected allocation top");
+    }
+    // Load allocation limit into ip. Result already contains allocation top.
+    ldr(ip, MemOperand(topaddr, limit - top));
   }
 
   // Calculate new top and bail out if new space is exhausted. Use result
-  // to calculate the new top. Object size is in words so a shift is required to
-  // get the number of bytes
-  ExternalReference new_space_allocation_limit =
-      ExternalReference::new_space_allocation_limit_address();
-  mov(scratch2, Operand(new_space_allocation_limit));
-  ldr(scratch2, MemOperand(scratch2));
+  // to calculate the new top. Object size may be in words so a shift is
+  // required to get the number of bytes.
   if ((flags & SIZE_IN_WORDS) != 0) {
-    add(result, result, Operand(object_size, LSL, kPointerSizeLog2));
+    add(scratch2, result, Operand(object_size, LSL, kPointerSizeLog2));
   } else {
-    add(result, result, Operand(object_size));
+    add(scratch2, result, Operand(object_size));
   }
-  cmp(result, Operand(scratch2));
+  cmp(scratch2, Operand(ip));
   b(hi, gc_required);
 
   // Update allocation top. result temporarily holds the new top.
   if (FLAG_debug_code) {
-    tst(result, Operand(kObjectAlignmentMask));
+    tst(scratch2, Operand(kObjectAlignmentMask));
     Check(eq, "Unaligned allocation in new space");
   }
-  str(result, MemOperand(scratch1));
+  str(scratch2, MemOperand(topaddr));
 
-  // Adjust back to start of new object.
-  if ((flags & SIZE_IN_WORDS) != 0) {
-    sub(result, result, Operand(object_size, LSL, kPointerSizeLog2));
-  } else {
-    sub(result, result, Operand(object_size));
-  }
-
   // Tag object if requested.
   if ((flags & TAG_OBJECT) != 0) {
     add(result, result, Operand(kHeapObjectTag));