Simplify uniform padding in Metal (Ganesh).

Previously, we would pad Metal uniforms to the nearest 16-byte size if
they contained a float3, float4, or matrix type. This does not appear
to be necessary (all tests pass without this level of padding).

Since Metal is C++ based, it does have *some* struct padding, based on
the basic type in the structure with the highest bit-width. Rather
than track this amount, we just assume that it is 8 and round up
Metal uniform blocks to the nearest 8-byte size. This will ~never be
larger than our previous padding, since a typical Skia shader will
generally always include a float2 uniform or larger (e.g. RTFlip is
a float2), and will probably be tighter than before since most shaders
include a color (float3/float4) or matrix uniform.

Change-Id: Ic8dd49f33cb81a24a6415e9ba6e91c9f6faeb1b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/556216
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

src/gpu/ganesh/mtl/GrMtlPipelineStateBuilder.mm

@@ -381,12 +381,15 @@ static MTLRenderPipelineColorAttachmentDescriptor* create_color_attachment(
     return mtlColorAttachment;
 }
 
-static uint32_t buffer_size(uint32_t offset, uint32_t maxAlignment) {
+static uint32_t buffer_size(uint32_t offset) {
-    // Metal expects the buffer to be padded at the end according to the alignment
+    // Metal uses C++ padding rules, so we round up the buffer size if it's not evenly divisible by
-    // of the largest element in the buffer.
+    // eight. The padding is dictated by the highest-bit-width basic type in the struct.
-    uint32_t offsetDiff = offset & maxAlignment;
+    // In practice, this will probably be four (float), but on the off chance we ever use a double
+    // or a pointer, we pad to eight.
+    constexpr uint32_t kMaxAlignment = 7;
+    uint32_t offsetDiff = offset & kMaxAlignment;
     if (offsetDiff != 0) {
-        offsetDiff = maxAlignment - offsetDiff + 1;
+        offsetDiff = kMaxAlignment - offsetDiff + 1;
     }
     return offset + offsetDiff;
 }
@@ -713,14 +716,12 @@ GrMtlPipelineState* GrMtlPipelineStateBuilder::finalize(
 
     sk_sp<GrMtlRenderPipeline> renderPipeline = GrMtlRenderPipeline::Make(pipelineState);
 
-    uint32_t bufferSize = buffer_size(fUniformHandler.fCurrentUBOOffset,
-                                      fUniformHandler.fCurrentUBOMaxAlignment);
     return new GrMtlPipelineState(fGpu,
                                   std::move(renderPipeline),
                                   pipelineDescriptor.colorAttachments[0].pixelFormat,
                                   fUniformHandles,
                                   fUniformHandler.fUniforms,
-                                  bufferSize,
+                                  buffer_size(fUniformHandler.fCurrentUBOOffset),
                                   (uint32_t)fUniformHandler.numSamplers(),
                                   std::move(fGPImpl),
                                   std::move(fXPImpl),

src/gpu/ganesh/mtl/GrMtlUniformHandler.h

@@ -58,9 +58,7 @@ private:
         : INHERITED(program)
         , fUniforms(kUniformsPerBlock)
         , fSamplers(kUniformsPerBlock)
-        , fCurrentUBOOffset(0)
+        , fCurrentUBOOffset(0) {}
-        , fCurrentUBOMaxAlignment(0x0) {
-    }
 
     UniformHandle internalAddUniformArray(const GrProcessor* owner,
                                           uint32_t visibility,
@@ -98,7 +96,6 @@ private:
     SkTArray<skgpu::Swizzle> fSamplerSwizzles;
 
     uint32_t            fCurrentUBOOffset;
-    uint32_t            fCurrentUBOMaxAlignment;
 
     friend class GrMtlPipelineStateBuilder;
 

src/gpu/ganesh/mtl/GrMtlUniformHandler.mm

@@ -156,13 +156,9 @@ static inline uint32_t sksltype_to_mtl_size(SkSLType type) {
 // taking into consideration all alignment requirements. The uniformOffset is set to the offset for
 // the new uniform, and currentOffset is updated to be the offset to the end of the new uniform.
 static uint32_t get_ubo_aligned_offset(uint32_t* currentOffset,
-                                       uint32_t* maxAlignment,
                                        SkSLType type,
                                        int arrayCount) {
     uint32_t alignmentMask = sksltype_to_alignment_mask(type);
-    if (alignmentMask > *maxAlignment) {
-        *maxAlignment = alignmentMask;
-    }
     uint32_t offsetDiff = *currentOffset & alignmentMask;
     if (offsetDiff != 0) {
         offsetDiff = alignmentMask - offsetDiff + 1;
@@ -200,8 +196,7 @@ GrGLSLUniformHandler::UniformHandle GrMtlUniformHandler::internalAddUniformArray
     }
     SkString resolvedName = fProgramBuilder->nameVariable(prefix, name, mangleName);
 
-    uint32_t offset = get_ubo_aligned_offset(&fCurrentUBOOffset, &fCurrentUBOMaxAlignment,
+    uint32_t offset = get_ubo_aligned_offset(&fCurrentUBOOffset, type, arrayCount);
-                                             type, arrayCount);
     SkString layoutQualifier;
     layoutQualifier.appendf("offset=%d", offset);