From 8b620227499cd843779a286a0d733b56022adcea Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Thu, 24 Dec 2020 11:36:05 +0100
Subject: [PATCH] backdrop: avoid a (benign) zero-sized read

Found with MEM_DEBUG added in later change.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 piet-gpu/shader/backdrop.comp |  20 +++++++++++---------
 piet-gpu/shader/backdrop.spv  | Bin 7752 -> 7840 bytes
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/piet-gpu/shader/backdrop.comp b/piet-gpu/shader/backdrop.comp
index f57d6e0..fb01d0d 100644
--- a/piet-gpu/shader/backdrop.comp
+++ b/piet-gpu/shader/backdrop.comp
@@ -85,16 +85,18 @@ void main() {
                 el_ix = probe;
             }
         }
-        uint seq_ix = row - (el_ix > 0 ? sh_row_count[el_ix - 1] : 0);
         uint width = sh_row_width[el_ix];
-        // Process one row sequentially
-        // Read backdrop value per tile and prefix sum it
-        uint tile_el_ix = sh_row_base[el_ix] + seq_ix * 2 * width;
-        uint sum = memory[tile_el_ix];
-        for (uint x = 1; x < width; x++) {
-            tile_el_ix += 2;
-            sum += memory[tile_el_ix];
-            memory[tile_el_ix] = sum;
+        if (width > 0) {
+            // Process one row sequentially
+            // Read backdrop value per tile and prefix sum it
+            uint seq_ix = row - (el_ix > 0 ? sh_row_count[el_ix - 1] : 0);
+            uint tile_el_ix = sh_row_base[el_ix] + seq_ix * 2 * width;
+            uint sum = memory[tile_el_ix];
+            for (uint x = 1; x < width; x++) {
+                tile_el_ix += 2;
+                sum += memory[tile_el_ix];
+                memory[tile_el_ix] = sum;
+            }
         }
     }
 }
diff --git a/piet-gpu/shader/backdrop.spv b/piet-gpu/shader/backdrop.spv
index 72b24bf01fb41395717af80456da0e14334f2038..e71fd9b5c06cb42cf09a8c4cd09eb3ea34054251 100644
GIT binary patch
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