Revert "Allow FPs to elevate default precision for the entire fragment program"

This reverts commit 92d7ccafdf896cf19764e025873d13315a76842d.

Reason for revert: Vulkan errors.

Original change's description:
> Allow FPs to elevate default precision for the entire fragment program
> 
> Currently, GrConfigConversionEffect is able to round-trip on many mobile
> GPUs because it uses highp for all intermediate variables (including the
> texture fetch result). Separating the texture sample into a different
> processor breaks that.
> 
> This is a blunt instrument, not to be used lightly.
> 
> Bug: skia:
> Change-Id: I2ab365e3da79628069e2eb727c43c2bf45bfd789
> Reviewed-on: https://skia-review.googlesource.com/10162
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> 

TBR=bsalomon@google.com,robertphillips@google.com,brianosman@google.com,ethannicholas@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: Iee5bb409f86a9cabecc76bd1273a5b3cef6af179
Reviewed-on: https://skia-review.googlesource.com/10967
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

2 files changed, 5 insertions, 10 deletions

diff --git a/include/gpu/GrTypesPriv.h b/include/gpu/GrTypesPriv.h
index cbff7d8a3b..5db1c24765 100644
--- a/include/gpu/GrTypesPriv.h
+++ b/include/gpu/GrTypesPriv.h
@@ -116,14 +116,11 @@ enum GrSLPrecision {
     kMedium_GrSLPrecision,
     kHigh_GrSLPrecision,
 
-    // Default precision is a special tag that means "whatever the default for the program/type
-    // combination is". In other words, it maps to the empty string in shader code. There are some
-    // scenarios where kDefault is not allowed (as the default precision for a program, or for
-    // varyings, for example).
-    kDefault_GrSLPrecision,
-
-    // We only consider the "real" precisions here
-    kLast_GrSLPrecision = kHigh_GrSLPrecision,
+    // Default precision is medium. This is because on OpenGL ES 2 highp support is not
+    // guaranteed. On (non-ES) OpenGL the specifiers have no effect on precision.
+    kDefault_GrSLPrecision = kMedium_GrSLPrecision,
+
+    kLast_GrSLPrecision = kHigh_GrSLPrecision
 };
 
 static const int kGrSLPrecisionCount = kLast_GrSLPrecision + 1;
diff --git a/include/private/GrGLSL.h b/include/private/GrGLSL.h
index 66f7be84d1..ed33c1cf4c 100644
--- a/include/private/GrGLSL.h
+++ b/include/private/GrGLSL.h
@@ -74,8 +74,6 @@ static inline const char* GrGLSLPrecisionString(GrSLPrecision p) {
             return "mediump";
         case kHigh_GrSLPrecision:
             return "highp";
-        case kDefault_GrSLPrecision:
-            return "";
         default:
             SkFAIL("Unexpected precision type.");
             return "";