X64: Make smi memory operations work directly on the embedded value.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 3d03949f25633f7cdefbfe1b5de5801b26bee995..8bdce9cb09cd440eab6d0058c62f7d5deb557c4f 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -239,6 +239,49 @@ Operand::Operand(Register index,
 }
 
 
+Operand::Operand(const Operand& operand, int32_t offset) {
+  ASSERT(operand.len_ >= 1);
+  // Operand encodes REX ModR/M [SIB] [Disp].
+  byte modrm = operand.buf_[0];
+  ASSERT(modrm < 0xC0);  // Disallow mode 3 (register target).
+  bool has_sib = ((modrm & 0x07) == 0x04);
+  byte mode = modrm & 0xC0;
+  int disp_offset = has_sib ? 2 : 1;
+  int base_reg = (has_sib ? operand.buf_[1] : modrm) & 0x07;
+  // Mode 0 with rbp/r13 as ModR/M or SIB base register always has a 32-bit
+  // displacement.
+  bool is_baseless = (mode == 0) && (base_reg == 0x05); // No base or RIP base.
+  int32_t disp_value = 0;
+  if (mode == 0x80 || is_baseless) {
+    // Mode 2 or mode 0 with rbp/r13 as base: Word displacement.
+    disp_value = *reinterpret_cast<const int32_t*>(&operand.buf_[disp_offset]);
+  } else if (mode == 0x40) {
+    // Mode 1: Byte displacement.
+    disp_value = static_cast<signed char>(operand.buf_[disp_offset]);
+  }
+  // Write new operand with same registers, but with modified displacement.

[William Hesse, 2010/05/26 08:13:24]

Insert blank line after reading and before writing, to separate the two parts of
the function.

[Lasse Reichstein, 2010/05/26 08:34:11]

Done.

+  disp_value += offset;

[William Hesse, 2010/05/26 08:13:24]

The assert for no overflow could be added:
ASSERT(disp_value <= 0 || offset <= 0 || disp_value + offset > 0);
ASSERT(disp_value >= 0 || offset >=0 || disp_value + offset < 0);

[Lasse Reichstein, 2010/05/26 08:34:11]

Assertion added.

+  rex_ = operand.rex_;
+  if (!is_int8(disp_value) || is_baseless) {
+    // Need 32 bits of displacement, mode 2 or mode 1 with register rbp/r13.
+    buf_[0] = (modrm & 0x3f) | (is_baseless ? 0x00 : 0x80);
+    len_ = disp_offset + 4;
+    Memory::int32_at(&buf_[disp_offset]) = disp_value;
+  } else if (disp_value != 0 || (base_reg == 0x05)) {
+    // Need 8 bits of displacement.
+    buf_[0] = (modrm & 0x3f) | 0x40;  // Mode 1.
+    len_ = disp_offset + 1;
+    buf_[disp_offset] = static_cast<byte>(disp_value);
+  } else {
+    // Need no displacement.
+    buf_[0] = (modrm & 0x3f);  // Mode 0.
+    len_ = disp_offset;
+  }
+  if (has_sib) {
+    buf_[1] = operand.buf_[1];
+  }
+}
+
 // -----------------------------------------------------------------------------
 // Implementation of Assembler.