[Pixman] [PATCH] float-combiner.c: Change tests for x == 0.0 tests to - FLT_MIN < x < FLT_MIN

[Søren Sandmann]

From: Søren Sandmann Pedersen <ssp at redhat.com>

pixman-float-combiner.c currently uses checks like these:

    if (x == 0.0f)
        ...
    else
        ... / x;

to prevent division by 0. In theory this is correct: a division-by-zero
exception is only supposed to happen when the floating point numerator is
exactly equal to a positive or negative zero.

However, in practice, the combination of x87 and gcc optimizations
causes issues. The x87 registers are 80 bits wide, which means the
initial test:

	if (x == 0.0f)

may be false when x is an 80 bit floating point number, but when x is
rounded to a 32 bit single precision number, it becomes equal to
0.0. In principle, gcc should compensate for this quirk of x87, and
there are some options such as -ffloat-store, -fexcess-precision=standard,
and -std=c99 that will make it do so, but these all have a performance
cost.  It is also possible to set the FPU to a mode that makes it do
all computation with single or double precision, but that would
require pixman to save the existing mode before doing anything with
floating point and restore it afterwards.

Instead, this patch side-steps the issue by replacing exact checks for
equality with zero with a new macro that checkes whether the value is
between -FLT_MIN and FLT_MIN.

There is extensive reading material about this issue linked off the
infamous gcc bug 323:

    http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
---
 pixman/pixman-combine-float.c |   28 +++++++++++++++-------------
 1 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/pixman/pixman-combine-float.c b/pixman/pixman-combine-float.c
index c3d54f0..c916df8 100644
--- a/pixman/pixman-combine-float.c
+++ b/pixman/pixman-combine-float.c
@@ -42,6 +42,8 @@
 #define force_inline __inline__
 #endif
 
+#define IS_ZERO(f)     (-FLT_MIN < (f) && (f) < FLT_MIN)
+
 typedef float (* combine_channel_t) (float sa, float s, float da, float d);
 
 static force_inline void
@@ -201,56 +203,56 @@ get_factor (combine_factor_t factor, float sa, float da)
 	break;
 
     case SA_OVER_DA:
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	    f = 1.0f;
 	else
 	    f = CLAMP (sa / da);
 	break;
 
     case DA_OVER_SA:
-	if (sa == 0.0f)
+	if (IS_ZERO (sa))
 	    f = 1.0f;
 	else
 	    f = CLAMP (da / sa);
 	break;
 
     case INV_SA_OVER_DA:
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	    f = 1.0f;
 	else
 	    f = CLAMP ((1.0f - sa) / da);
 	break;
 
     case INV_DA_OVER_SA:
-	if (sa == 0.0f)
+	if (IS_ZERO (sa))
 	    f = 1.0f;
 	else
 	    f = CLAMP ((1.0f - da) / sa);
 	break;
 
     case ONE_MINUS_SA_OVER_DA:
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	    f = 0.0f;
 	else
 	    f = CLAMP (1.0f - sa / da);
 	break;
 
     case ONE_MINUS_DA_OVER_SA:
-	if (sa == 0.0f)
+	if (IS_ZERO (sa))
 	    f = 0.0f;
 	else
 	    f = CLAMP (1.0f - da / sa);
 	break;
 
     case ONE_MINUS_INV_DA_OVER_SA:
-	if (sa == 0.0f)
+	if (IS_ZERO (sa))
 	    f = 0.0f;
 	else
 	    f = CLAMP (1.0f - (1.0f - da) / sa);
 	break;
 
     case ONE_MINUS_INV_SA_OVER_DA:
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	    f = 0.0f;
 	else
 	    f = CLAMP (1.0f - (1.0f - sa) / da);
@@ -403,11 +405,11 @@ blend_lighten (float sa, float s, float da, float d)
 static force_inline float
 blend_color_dodge (float sa, float s, float da, float d)
 {
-    if (d == 0.0f)
+    if (IS_ZERO (d))
 	return 0.0f;
     else if (d * sa >= sa * da - s * da)
 	return sa * da;
-    else if (sa - s == 0.0f)
+    else if (IS_ZERO (sa - s))
 	return sa * da;
     else
 	return sa * sa * d / (sa - s);
@@ -420,7 +422,7 @@ blend_color_burn (float sa, float s, float da, float d)
 	return sa * da;
     else if (sa * (da - d) >= s * da)
 	return 0.0f;
-    else if (s == 0.0f)
+    else if (IS_ZERO (s))
 	return 0.0f;
     else
 	return sa * (da - sa * (da - d) / s);
@@ -440,14 +442,14 @@ blend_soft_light (float sa, float s, float da, float d)
 {
     if (2 * s < sa)
     {
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	    return d * sa;
 	else
 	    return d * sa - d * (da - d) * (sa - 2 * s) / da;
     }
     else
     {
-	if (da == 0.0f)
+	if (IS_ZERO (da))
 	{
 	    return 0.0f;
 	}
-- 
1.7.4