Merge pull request 'perf(ui): skip the destination read-modify-write on empty tiles (#127)' (#144) from claude/issue-127 into master

shaders/ui-circles.comp.glsl

@@ -23,12 +23,16 @@ void main() {
     ivec2 screenPx;
     bool  valid = uiResolveScreenPixel(pc.hdr, screenPx);
 
+    // Defer the destination read-modify-write: a sparse UI leaves most tiles
+    // untouched, so only load the pixel when the first surviving item blends
+    // over it (`loaded`), and only store when something actually touched it.
+    // The fused kernel amortizes a single load/store across all categories; the
+    // standalone Dispatch* path has no such umbrella and otherwise pays a full
+    // read-modify-write per empty tile.
     vec4 dst    = vec4(0.0);
     vec2 sp     = vec2(0.0);
-    if (valid) {
+    bool loaded = false;
-        dst = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+    if (valid) sp = vec2(screenPx) + 0.5;
-        sp  = vec2(screenPx) + 0.5;
-    }
 
     vec2 tileMin, tileMax;
     uiTileBounds(tileMin, tileMax);
@@ -90,11 +94,15 @@ void main() {
                 }
 
                 if (src.a <= 0.0) continue;
+                if (!loaded) {
+                    dst    = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+                    loaded = true;
+                }
                 dst = uiBlendOver(dst, src);
             }
         }
         barrier();
     }
 
-    if (valid) imageStore(uiImages[pc.hdr.outImage], screenPx, dst);
+    if (loaded) imageStore(uiImages[pc.hdr.outImage], screenPx, dst); // loaded ⇒ valid
 }

shaders/ui-images.comp.glsl

@@ -29,12 +29,16 @@ void main() {
     ivec2 screenPx;
     bool  valid = uiResolveScreenPixel(pc.hdr, screenPx);
 
+    // Defer the destination read-modify-write: a sparse UI leaves most tiles
+    // untouched, so only load the pixel when the first surviving item blends
+    // over it (`loaded`), and only store when something actually touched it.
+    // The fused kernel amortizes a single load/store across all categories; the
+    // standalone Dispatch* path has no such umbrella and otherwise pays a full
+    // read-modify-write per empty tile.
     vec4 dst    = vec4(0.0);
     vec2 sp     = vec2(0.0);
-    if (valid) {
+    bool loaded = false;
-        dst = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+    if (valid) sp = vec2(screenPx) + 0.5;
-        sp  = vec2(screenPx) + 0.5;
-    }
 
     vec2 tileMin, tileMax;
     uiTileBounds(tileMin, tileMax);
@@ -86,11 +90,15 @@ void main() {
                 );
                 vec4 src = sampled * s_tint[c];
                 if (src.a <= 0.0) continue;
+                if (!loaded) {
+                    dst    = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+                    loaded = true;
+                }
                 dst = uiBlendOver(dst, src);
             }
         }
         barrier();
     }
 
-    if (valid) imageStore(uiImages[pc.hdr.outImage], screenPx, dst);
+    if (loaded) imageStore(uiImages[pc.hdr.outImage], screenPx, dst); // loaded ⇒ valid
 }

shaders/ui-quads.comp.glsl

@@ -27,12 +27,16 @@ void main() {
     ivec2 screenPx;
     bool  valid = uiResolveScreenPixel(pc.hdr, screenPx);
 
+    // Defer the destination read-modify-write: a sparse UI leaves most tiles
+    // untouched, so only load the pixel when the first surviving item blends
+    // over it (`loaded`), and only store when something actually touched it.
+    // The fused kernel amortizes a single load/store across all categories; the
+    // standalone Dispatch* path has no such umbrella and otherwise pays a full
+    // read-modify-write per empty tile.
     vec4 dst    = vec4(0.0);
     vec2 sp     = vec2(0.0);
-    if (valid) {
+    bool loaded = false;
-        dst = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+    if (valid) sp = vec2(screenPx) + 0.5;
-        sp  = vec2(screenPx) + 0.5;
-    }
 
     vec2 tileMin, tileMax;
     uiTileBounds(tileMin, tileMax);
@@ -93,11 +97,15 @@ void main() {
                 }
 
                 if (src.a <= 0.0) continue;
+                if (!loaded) {
+                    dst    = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+                    loaded = true;
+                }
                 dst = uiBlendOver(dst, src);
             }
         }
         barrier(); // done reading shared for this chunk before it's overwritten
     }
 
-    if (valid) imageStore(uiImages[pc.hdr.outImage], screenPx, dst);
+    if (loaded) imageStore(uiImages[pc.hdr.outImage], screenPx, dst); // loaded ⇒ valid
 }

shaders/ui-text.comp.glsl

@@ -38,12 +38,16 @@ void main() {
     ivec2 screenPx;
     bool  valid = uiResolveScreenPixel(pc.hdr, screenPx);
 
+    // Defer the destination read-modify-write: a sparse UI leaves most tiles
+    // untouched, so only load the pixel when the first surviving glyph blends
+    // over it (`loaded`), and only store when something actually touched it.
+    // The fused kernel amortizes a single load/store across all categories; the
+    // standalone Dispatch* path has no such umbrella and otherwise pays a full
+    // read-modify-write per empty tile.
     vec4 dst    = vec4(0.0);
     vec2 sp     = vec2(0.0);
-    if (valid) {
+    bool loaded = false;
-        dst = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+    if (valid) sp = vec2(screenPx) + 0.5;
-        sp  = vec2(screenPx) + 0.5;
-    }
 
     vec2 tileMin, tileMax;
     uiTileBounds(tileMin, tileMax);
@@ -110,11 +114,15 @@ void main() {
 
                 vec4 col = s_color[c];
                 vec4 src = vec4(col.rgb, col.a * a);
+                if (!loaded) {
+                    dst    = imageLoad(uiImages[pc.hdr.outImage], screenPx);
+                    loaded = true;
+                }
                 dst = uiBlendOver(dst, src);
             }
         }
         barrier();
     }
 
-    if (valid) imageStore(uiImages[pc.hdr.outImage], screenPx, dst);
+    if (loaded) imageStore(uiImages[pc.hdr.outImage], screenPx, dst); // loaded ⇒ valid
 }