Optimize the assembly code generated for Math.random()

src/ia32/codegen-ia32.cc

Index: src/ia32/codegen-ia32.cc
===================================================================
--- src/ia32/codegen-ia32.cc	(revision 4383)
+++ src/ia32/codegen-ia32.cc	(working copy)
@@ -6438,7 +6438,7 @@
   Label slow_allocate_heapnumber;
   Label heapnumber_allocated;
 
-  __ AllocateHeapNumber(eax, ebx, ecx, &slow_allocate_heapnumber);
+  __ AllocateHeapNumber(edi, ebx, ecx, &slow_allocate_heapnumber);
   __ jmp(&heapnumber_allocated);
 
   __ bind(&slow_allocate_heapnumber);
@@ -6447,14 +6447,39 @@
   // -0.0.  A new, distinct heap number is returned each time.
   __ push(Immediate(Smi::FromInt(0)));
   __ CallRuntime(Runtime::kNumberUnaryMinus, 1);
+  __ mov(edi, eax);
 
   __ bind(&heapnumber_allocated);
 
-  __ PrepareCallCFunction(1, ebx);
-  __ mov(Operand(esp, 0), eax);
-  __ CallCFunction(ExternalReference::fill_heap_number_with_random_function(),
-                   1);
+  __ PrepareCallCFunction(0, ebx);
+  __ CallCFunction(ExternalReference::random_uint32_function(), 0);
 
+  // Convert 32 random bits in eax to 0.(32 random bits) in a double
+  // by computing:
+  // ( 1.(20 0s)(32 random bits) x 2^20 ) - (1.0 x 2^20)).
+  // This is implemented on both SSE2 and FPU.
+  if (CpuFeatures::IsSupported(SSE2)) {
+    CpuFeatures::Scope fscope(SSE2);
+    __ mov(ebx, Immediate(0x49800000));  // 1.0 x 2^20 as single.
+    __ movd(xmm1, Operand(ebx));
+    __ movd(xmm0, Operand(eax));
+    __ cvtss2sd(xmm1, xmm1);
+    __ pxor(xmm0, xmm1);
+    __ subsd(xmm0, xmm1);
+    __ movdbl(FieldOperand(edi, HeapNumber::kValueOffset), xmm0);
+  } else {
+    // 0x4130000000000000 is 1.0 x 2^20 as a double.
+    __ mov(FieldOperand(edi, HeapNumber::kExponentOffset),
+           Immediate(0x41300000));
+    __ mov(FieldOperand(edi, HeapNumber::kMantissaOffset), eax);
+    __ fld_d(FieldOperand(edi, HeapNumber::kValueOffset));
+    __ mov(FieldOperand(edi, HeapNumber::kMantissaOffset), Immediate(0));
+    __ fld_d(FieldOperand(edi, HeapNumber::kValueOffset));
+    __ fsubp(1);
+    __ fstp_d(FieldOperand(edi, HeapNumber::kValueOffset));
+  }
+  __ mov(eax, edi);
+
   Result result = allocator_->Allocate(eax);
   frame_->Push(&result);
 }