Some inline ARM assembly for saturated arithmetic, a small speed-up for

Source/wtf/SaturatedArithmetic.h

Index: Source/wtf/SaturatedArithmetic.h
diff --git a/Source/wtf/SaturatedArithmetic.h b/Source/wtf/SaturatedArithmetic.h
index 981276f7c484fa779d5a30869670081984e0bfd8..fe3ff3c383dc8fdfc5a2b9900ae93ad44fd3cf5f 100644
--- a/Source/wtf/SaturatedArithmetic.h
+++ b/Source/wtf/SaturatedArithmetic.h
@@ -31,19 +31,30 @@
 #ifndef SaturatedArithmetic_h
 #define SaturatedArithmetic_h
 
+#include "wtf/CPU.h"
 #include <limits>
 #include <stdint.h>
 
+#if CPU(ARM_TRADITIONAL) && COMPILER(GCC)
+
+// If we're building ARM on GCC we replace the C++ versions with some
+// native arm assembly for speed.
+#include "wtf/asm/SaturatedArithmeticAsm.h"

[eseidel, 2014/06/20 16:30:20]

I probably would have called it wtf/SaturatedArithmaticARM.h, or something to
indicate what flavor of ASM it is.

[picksi, 2014/06/23 15:30:43]

Done.

+
+#else
+
 ALWAYS_INLINE int32_t saturatedAddition(int32_t a, int32_t b)
 {
     uint32_t ua = a;
     uint32_t ub = b;
     uint32_t result = ua + ub;
 
-    // Can only overflow if the signed bit of the two values match. If the signed
-    // bit of the result and one of the values differ it did overflow.
-    if (!((ua ^ ub) >> 31) & (result ^ ua) >> 31)
+    // Can only overflow if the signed bit of the two values match. If the
+    // signed bit of the result and one of the values differ it overflowed.
+
+    if (~(ua ^ ub) & (result ^ ua) & (1 << 31)) {
         result = std::numeric_limits<int>::max() + (ua >> 31);

[Stephen Chennney, 2014/06/20 14:46:56]

No braces and immediately "return std::num..."

[picksi, 2014/06/23 15:30:43]

Done.

+    }
 
     return result;
 }
@@ -54,12 +65,60 @@ ALWAYS_INLINE int32_t saturatedSubtraction(int32_t a, int32_t b)
     uint32_t ub = b;
     uint32_t result = ua - ub;
 
-    // Can only overflow if the signed bit of the two values do not match. If the
-    // signed bit of the result and the first value differ it did overflow.
-    if ((ua ^ ub) >> 31 & (result ^ ua) >> 31)
+    // Can only overflow if the signed bit of the two input values differ. If
+    // the signed bit of the result and the first value differ it overflowed.
+
+    if ((ua ^ ub) & (result ^ ua) & (1 << 31)) {
         result = std::numeric_limits<int>::max() + (ua >> 31);

[Stephen Chennney, 2014/06/20 14:46:56]

Ditto.

[picksi, 2014/06/23 15:30:43]

Done.

+    }
 
     return result;
 }
 
+inline int getMaxSaturatedSetResultForTesting(int FractionalShift)
+{
+    // For C version the set function maxes out to max int, this differs from
+    // the ARM asm version, see SaturatedArithmetiAsm.h for the equivlent asm
+    // version.
+    return std::numeric_limits<int>::max();
+}
+
+inline int getMinSaturatedSetResultForTesting(int FractionalShift)
+{
+    return std::numeric_limits<int>::min();
+}
+
+ALWAYS_INLINE int saturatedSet(int value, int FractionalShift)
+{
+    const int intMaxForLayoutUnit =
+        std::numeric_limits<int>::max() >> FractionalShift;
+
+    const int intMinForLayoutUnit =
+        std::numeric_limits<int>::min() >> FractionalShift;
+
+    if (value >= intMaxForLayoutUnit) {
+        return std::numeric_limits<int>::max();
+    }

[Stephen Chennney, 2014/06/20 14:46:56]

No braces on single line statements in Blink.

[picksi, 2014/06/23 15:30:43]

Done.

+
+    if (value <= intMinForLayoutUnit) {
+        return std::numeric_limits<int>::min();
+    }

[Stephen Chennney, 2014/06/20 14:46:56]

Ditto.

[picksi, 2014/06/23 15:30:43]

Done.

+
+    return value << FractionalShift;
+}
+
+
+ALWAYS_INLINE int saturatedSet(unsigned value, int FractionalShift)
+{
+    const unsigned intMaxForLayoutUnit =
+        std::numeric_limits<int>::max() >> FractionalShift;
+
+    if (value >= intMaxForLayoutUnit) {
+        return std::numeric_limits<int>::max();
+    }

[Stephen Chennney, 2014/06/20 14:46:56]

Ditto,

[picksi, 2014/06/23 15:30:43]

Done.

+
+    return value << FractionalShift;
+}
+
+#endif // CPU(ARM_TRADITIONAL) && COMPILER(GCC)
 #endif // SaturatedArithmetic_h