finish up constants

For whatever reason, if I swap the condition in the if_then_else tests
from < to >= and swap the then/else values, I can use constants in
hsl_to_rgb.  Still don't understand why, but I'll take it.  I suspect it
has something to do with SSE, IEEE, and NaN, but I don't care enough to
speculate any more concretely.

This does that, removes C() and _f, updates some comments, and adds a
guard in build_stages.py to yell if it sees trouble like LCPI40_4...

This reminds me to try -ffast-math soon.  I think that was mostly held
back by constants.

Change-Id: I3f8a37a4d4642f77422ce3261b750061e9e604a3
Reviewed-on: https://skia-review.googlesource.com/14942
Reviewed-by: Herb Derby <herb@google.com>

src/jumper/SkJumper.h

@@ -48,17 +48,6 @@
     #include <stdint.h>
 #endif
 
-// SkJumper_stages.cpp has some unusual constraints on what constants it can use.
-//
-// If the constant is baked into the instruction, that's ok.
-// If the constant is synthesized through code, that's ok.
-// If the constant is loaded from memory, that's no good.
-//
-// We offer a couple facilities to get at any other constants you need:
-//   - the C() function usually constrains constants to be directly baked into an instruction; or
-//   - the _i and _f user-defined literal operators call C() for you in a prettier way; or
-//   - you can load values from this struct.
-
 static const int SkJumper_kMaxStride = 8;
 
 struct SkJumper_constants {

src/jumper/SkJumper_misc.h

@@ -36,17 +36,4 @@ SI Dst widen_cast(const Src& src) {
     return dst;
 }
 
-// A couple functions for embedding constants directly into code,
-// so that no .const or .literal4 section is created.
-SI float C(float f) {
-    int x = bit_cast<int>(f);
-#if defined(JUMPER) && defined(__x86_64__)
-    // Move x-the-compile-time-constant as a literal into x-the-register.
-    asm("mov %1, %0" : "=r"(x) : "i"(x));
-#endif
-    return bit_cast<float>(x);
-}
-// Syntax sugar to make C() easy to use for constant literals.
-SI float operator "" _f(long double f) { return C((float)f); }
-
 #endif//SkJumper_misc_DEFINED

src/jumper/SkJumper_stages.cpp

@@ -6,7 +6,7 @@
  */
 
 #include "SkJumper.h"
-#include "SkJumper_misc.h"     // SI, unaligned_load(), bit_cast(), C(), operator"" and _f.
+#include "SkJumper_misc.h"     // SI, unaligned_load(), bit_cast()
 #include "SkJumper_vectors.h"  // F, I32, U32, U16, U8, cast(), expand()
 
 // Our fundamental vector depth is our pixel stride.
@@ -17,9 +17,7 @@ static const size_t kStride = sizeof(F) / sizeof(float);
 // and that F, I32, etc. are kStride-deep ext_vector_types of the appropriate type.
 // Otherwise, F, I32, etc. just alias the basic scalar types (and so kStride == 1).
 
-// Another reminder:
-// You can't generally use constants in this file except via C() or operator"" _f.
-// Not all constants can be generated using C() or _f.  Stages read the rest from this struct.
+// You can use most constants in this file, but in a few rare exceptions we read from this struct.
 using K = const SkJumper_constants;
 
 
@@ -504,16 +502,17 @@ STAGE(hsl_to_rgb) {
       s = g,
       l = b;
 
-    F q = l + if_then_else(l < 0.5_f,     l*s
-                                    , s - l*s);
-    F p = 2.0f*l - q;
+    F q = l + if_then_else(l >= 0.5f, s - l*s, l*s),
+      p = 2.0f*l - q;
 
     auto hue_to_rgb = [&](F t) {
         t = fract(t);
-        return if_then_else(t < C(1/6.0f),  p + (q-p)*(       6.0f*t),
-               if_then_else(t < C(3/6.0f),  q,
-               if_then_else(t < C(4/6.0f),  p + (q-p)*(4.0f - 6.0f*t),
-                                            p)));
+
+        F r = p;
+        r = if_then_else(t >= 4/6.0f, r, p + (q-p)*(4.0f - 6.0f*t));
+        r = if_then_else(t >= 3/6.0f, r, q);
+        r = if_then_else(t >= 1/6.0f, r, p + (q-p)*(       6.0f*t));
+        return r;
     };
 
     r = if_then_else(s == 0, l, hue_to_rgb(h + (1/3.0f)));

src/jumper/build_stages.py

@@ -115,6 +115,9 @@ def parse_object_file(dot_o, directive, target=None):
         print sym.replace('.literal', align)
       elif sym.startswith('.const'):  # 32-byte constants
         print align + '32'
+      elif not sym.startswith('sk_'):
+        print >>sys.stderr, "build_stages.py can't handle '%s' (yet?)." % sym
+        assert sym.startswith('sk_')
       else:  # a stage function
         if hidden:
           print hidden + ' _' + sym