MIPS: Improve runtime detection and compatibility wrt arch. revisions.

src/base/cpu.cc

Index: src/base/cpu.cc
diff --git a/src/base/cpu.cc b/src/base/cpu.cc
index fbfbcf683b74e3014257cfe749e96eabdf2203cf..0a737a66d488e090e611f40440f64e001708123b 100644
--- a/src/base/cpu.cc
+++ b/src/base/cpu.cc
@@ -119,13 +119,18 @@ static uint32_t ReadELFHWCaps() {
 int __detect_fp64_mode(void) {
   double result = 0;
   // Bit representation of (double)1 is 0x3FF0000000000000.
-  asm(
-    "lui $t0, 0x3FF0\n\t"
-    "ldc1 $f0, %0\n\t"
-    "mtc1 $t0, $f1\n\t"
-    "sdc1 $f0, %0\n\t"
-    : "+m" (result)
-    : : "t0", "$f0", "$f1", "memory");
+  __asm__ volatile(
+      ".set push\n\t"
+      ".set noreorder\n\t"
+      ".set oddspreg\n\t"
+      "lui $t0, 0x3FF0\n\t"
+      "ldc1 $f0, %0\n\t"
+      "mtc1 $t0, $f1\n\t"
+      "sdc1 $f0, %0\n\t"
+      ".set pop\n\t"
+      : "+m"(result)
+      :
+      : "t0", "$f0", "$f1", "memory");
 
   return !(result == 1);
 }
@@ -133,9 +138,33 @@ int __detect_fp64_mode(void) {
 
 int __detect_mips_arch_revision(void) {
   // TODO(dusmil): Do the specific syscall as soon as it is implemented in mips
-  // kernel. Currently fail-back to the least common denominator which is
-  // mips32 revision 1.
-  return 1;
+  // kernel.
+  // Detect r6 architectures through QNaN representation.
+  // The r6 variant uses IEEE2008 QNaN representation which is 7ff8xxxxxxxxxxxx,
+  // the same encoding for QNaN on x86, ARM.
+  // Non r6 variants uses 7ff7fxxxxxxxxxxx for QNaN which is SNaN on x86, ARM.
+  double result = 0;
+  uint64_t check, mask = 0x8000000000000LLU;
+  union {
+    double d;
+    uint64_t ull;
+  } conv;
+
+  __asm__ volatile(
+      ".set push\n\t"
+      ".set noreorder\n\t"
+      "ldc1 $f0, %0\n\t"
+      "div.d $f0, $f0, $f0\n\t"
+      "sdc1 $f0, %0\n\t"
+      ".set pop\n\t"
+      : "+m"(result)
+      :
+      : "$f0", "memory");
+  conv.d = result;
+  // Check bit 50 of NaN value.
+  // Fail-back to the least common denominator which is mips32 revision 1.

[paul.l..., 2014/10/02 16:26:47]

nit: this should probably be "fall-back"

[dusmil.imgtec, 2014/10/02 17:02:09]

Done.

+  check = conv.ull & mask;
+  return check ? 6 : 1;
 }
 #endif