Use Offset instead of ConstOffset for GLSL function textureGatherOffset (#5426)

* Use Offset instead of ConstOffset for GLSL function textureGatherOffset

This commit changes to use `Offset` option on OpImageGather instruction
when translating `textureGatherOffset` to SPIR-V code.

Interestingly GLSL allows the offset value to be a variable not
constant just for the function `textureGatherOffset` while all other
offset variants of texture sampling functions require the offset to be a
constant value.

From a few experiments, I found that the spirv-optimizer changes
`Offset` to `ConstOffset` if the offset is a compile-time value. I also
found that when the offset value is zero, it changes to `None` with no
offset value.

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/bugs/gh-5339.slang b/tests/bugs/gh-5339.slang
new file mode 100644
index 000000000..163b05bc7
--- /dev/null
+++ b/tests/bugs/gh-5339.slang
@@ -0,0 +1,27 @@
+//TEST:SIMPLE(filecheck=SPV): -allow-glsl -target spirv-asm -entry computeMain -stage compute
+
+// Test if we are correctly using `Offset` option instead of `ConstOffset`
+// when the offset value is not a compile-time constant.
+
+//SPV:OpCapability ImageGatherExtended
+
+#extension GL_EXT_gpu_shader5 : require
+
+layout (location = 0) in highp vec2 v_texCoord;
+
+layout (binding = 0) uniform highp sampler2D u_sampler;
+layout (binding = 1) uniform offset { highp ivec2 u_offset; };
+
+//TEST_INPUT:ubuffer(data=[0], stride=4):out,name=outputBuffer
+buffer MyBlockName
+{
+    vec4 result;
+} outputBuffer;
+
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    //SPV:OpImageGather %
+    //SPV-NOT:Const
+    //SPV-SAME: Offset %
+    outputBuffer.result = textureGatherOffset(u_sampler, v_texCoord, u_offset);
+}