[hlsl] PointCoord Support

[flibitijibibo]

I'm pretty sure we noticed this the first time around, but it came up again while we were using Flotilla as an experiment for NativeAOT support. Simply put, GLSL/SPIR-V both have gl_PointCoord or gl_TexCoord[0] working with point sprites, but D3D11 doesn't appear to have a straightforward equivalent (as far as I know, TEXCOORD0 doesn't work like it did with D3D9). Microsoft incorrectly suggests using VPOS, and ANGLE emulates point sprites with geometry shaders, so this doesn't look good.

[TheSpydog]

If anyone's interested in experimenting with possible solutions, I've scribbled out a rough version of what the general structure might look like, minus the key part -- what semantic or convoluted workaround we should use.

diff --git a/mojoshader_d3d11.c b/mojoshader_d3d11.c
index 92902fd..e113a05 100644
--- a/mojoshader_d3d11.c
+++ b/mojoshader_d3d11.c
@@ -415,7 +415,7 @@ static char *rewritePixelShader(MOJOSHADER_d3d11Shader *vshader,
     const char *_Input = "_Input" ENDLINE_STR "{" ENDLINE_STR;
     const char *vsrc = vpd->output;
     const char *psrc = ppd->output;
-    const char *a, *b, *vout, *pstart, *vface, *pend;
+    const char *a, *b, *vout, *pstart, *vface, *pointcoord, *pend;
     size_t substr_len;
     char *pfinal;

@@ -449,6 +449,7 @@ static char *rewritePixelShader(MOJOSHADER_d3d11Shader *vshader,
     // Find matching semantics
     int i, j;
     int vfaceidx = -1;
+    int pointcoordidx = -1;
     const char *pvarname, *vvarname;
     for (i = 0; i < ppd->attribute_count; i++)
     {
@@ -477,7 +478,21 @@ static char *rewritePixelShader(MOJOSHADER_d3d11Shader *vshader,
         } // for

         if (strcmp(ppd->attributes[i].name, "vFace") == 0)
+        {
+            found_matching_vs_output_for_ps_input = 1;
             vfaceidx = i;
+        } // if
+
+        if (!found_matching_vs_output_for_ps_input)
+        {
+            if (ppd->attributes[i].usage == MOJOSHADER_USAGE_TEXCOORD &&
+                ppd->attributes[i].index == 0)
+            {
+                found_matching_vs_output_for_ps_input = 1;
+                pointcoordidx = i;
+                replaceVarname(ppd->attributes[i].name, "pointCoord", &pend);
+            } // if
+        } // if

         // A vertex shader that doesn't properly initialize all its outputs
         //  can produce a situation where vpd->outputs is missing a matching
@@ -491,14 +506,18 @@ static char *rewritePixelShader(MOJOSHADER_d3d11Shader *vshader,
     // Special handling for VFACE
     vface = (vfaceidx != -1) ? "\tbool m_vFace : SV_IsFrontFace;\n" : "";

+    // Special handling for point coord (aka TEXCOORD0)
+    pointcoord = (pointcoordidx != -1) ? "\tfloat2 m_pointCoord : TEXCOORD0;\n" : ""; // FIXME: What semantic...?
+
     // Concatenate the shader pieces together
-    substr_len = strlen(pstart) + strlen(vout) + strlen(vface) + strlen(pend);
+    substr_len = strlen(pstart) + strlen(vout) + strlen(vface) + strlen(pointcoord) + strlen(pend);
     pfinal = (char *) ctx->malloc_fn(substr_len + 1, ctx->malloc_data);
     memset((void *) pfinal, '\0', substr_len + 1);
     memcpy(pfinal, pstart, strlen(pstart));
     memcpy(pfinal + strlen(pstart), vout, strlen(vout));
     memcpy(pfinal + strlen(pstart) + strlen(vout), vface, strlen(vface));
-    memcpy(pfinal + strlen(pstart) + strlen(vout) + strlen(vface), pend, strlen(pend));
+    memcpy(pfinal + strlen(pstart) + strlen(vout) + strlen(vface), pointcoord, strlen(pointcoord));
+    memcpy(pfinal + strlen(pstart) + strlen(vout) + strlen(vface) + strlen(pointcoord), pend, strlen(pend));

     // Free the temporary buffers
     ctx->free_fn((void *) vout, ctx->malloc_data);