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gl: native-wide perf collapse root cause - wide FBO lacked a stencil attachment

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Proven by ablation on Tomba2 16:9 attract (12fps): a full mirror pass cost
~616us GPU per pass, but ablate=2 (FBO binds + state churn, no draws) = 3us
and ablate=3 (identical draws into the stencil-equipped hr FBO) = 1.5us.
The only difference between the fast and catastrophic configurations is
drawing with stencil test/write enabled (every batch's mask fixup pass)
into the one FBO with no stencil attachment - the driver handles that
combination with ~0.6ms of per-draw work. This also explains why the
deferred once-per-frame replay experiment (ws branch a8181dd) did not help:
it still drew into the stencil-less wide FBO, so the per-draw cost stayed.

Fix: each wide surface gets a DEPTH24_STENCIL8 renderbuffer like the hr FBO.
This is also a correctness fix: the wide surface now carries a real PSX
mask-bit stencil mirror (previously mask set/check silently no-opped on the
wide pass). wide_clear clears the stencil to bit15 alongside the color, and
surface creation clears both.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Matthew Stanley
2026-07-03 20:26:53 -07:00
parent e9b4ed5bbf
commit cfa79bbc57
1 changed files with 26 additions and 3 deletions
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runtime/src/gpu_gl_renderer.c
+26 -3
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@@ -151,6 +151,7 @@ typedef void (APIENTRY *PFN_glFramebufferTexture2D)(GLenum, GLenum, GLenum, GL
typedef GLenum (APIENTRY *PFN_glCheckFramebufferStatus)(GLenum);
typedef void (APIENTRY *PFN_glBlitFramebuffer)(GLint,GLint,GLint,GLint,GLint,GLint,GLint,GLint,GLbitfield,GLenum);
typedef void (APIENTRY *PFN_glGenRenderbuffers)(GLsizei, GLuint *);
typedef void (APIENTRY *PFN_glDeleteRenderbuffers)(GLsizei, const GLuint *);
typedef void (APIENTRY *PFN_glBindRenderbuffer)(GLenum, GLuint);
typedef void (APIENTRY *PFN_glRenderbufferStorage)(GLenum, GLenum, GLsizei, GLsizei);
typedef void (APIENTRY *PFN_glFramebufferRenderbuffer)(GLenum, GLenum, GLenum, GLuint);
@@ -226,6 +227,7 @@ static void gl_perf_mirror_end(void); /* mirror pass (timestamp pair; splits s
static int s_ws_ablate = 0;
static void flush_tex_batch(void); /* textured-prim batch — defined below, flushed from coherency points */
static PFN_glGenRenderbuffers p_glGenRenderbuffers;
static PFN_glDeleteRenderbuffers p_glDeleteRenderbuffers;
static PFN_glBindRenderbuffer p_glBindRenderbuffer;
static PFN_glRenderbufferStorage p_glRenderbufferStorage;
static PFN_glFramebufferRenderbuffer p_glFramebufferRenderbuffer;
@@ -256,6 +258,7 @@ static int load_modern_gl(void) {
LOAD(p_glCheckFramebufferStatus, "glCheckFramebufferStatus");
LOAD(p_glBlitFramebuffer, "glBlitFramebuffer");
LOAD(p_glGenRenderbuffers, "glGenRenderbuffers");
LOAD(p_glDeleteRenderbuffers, "glDeleteRenderbuffers");
LOAD(p_glBindRenderbuffer, "glBindRenderbuffer");
LOAD(p_glRenderbufferStorage, "glRenderbufferStorage");
LOAD(p_glFramebufferRenderbuffer, "glFramebufferRenderbuffer");
@@ -402,6 +405,13 @@ static int s_tex_filter = 0;
#define WIDE_MAX_SURF 4
static GLuint s_wide_tex[WIDE_MAX_SURF]; /* color tex per surface (0 = free) */
static GLuint s_wide_fbo[WIDE_MAX_SURF]; /* FBO per surface */
static GLuint s_wide_rb[WIDE_MAX_SURF]; /* depth-stencil RB per surface (mask
* mirror, same as hr). PERF-CRITICAL:
* a stencil-less wide FBO made every
* stencil-enabled mirror draw cost
* ~0.6ms of driver work (Tomba2 16:9
* collapsed to 12fps); with the RB
* attached the pass costs ~2us. */
static int s_wide_base[WIDE_MAX_SURF]; /* base_x per surface (-1 = free) */
static int g_wide_w = 0; /* wide width (native px); 0 = disabled */
static int g_wide_off = 0; /* centering OFFSET (native px) */
@@ -2138,6 +2148,7 @@ static void wide_free_all(void) {
for (int i = 0; i < WIDE_MAX_SURF; i++) {
if (s_wide_fbo[i]) { p_glDeleteFramebuffers(1, &s_wide_fbo[i]); s_wide_fbo[i] = 0; }
if (s_wide_tex[i]) { glDeleteTextures(1, &s_wide_tex[i]); s_wide_tex[i] = 0; }
if (s_wide_rb[i]) { p_glDeleteRenderbuffers(1, &s_wide_rb[i]); s_wide_rb[i] = 0; }
s_wide_base[i] = -1;
}
g_wide_cur = 0;
@@ -2154,15 +2165,25 @@ static GLuint wide_fbo_for(int base_x) {
int w = g_wide_w * s_scale, h = VRAM_H * s_scale;
s_cw_fbo_creates++;
s_wide_tex[i] = make_tex(GL_RGBA8, w, h, GL_RGBA, GL_UNSIGNED_BYTE);
if (!make_fbo(&s_wide_fbo[i], s_wide_tex[i], 0)) {
/* Depth-stencil RB, same as the hr FBO: the stencil carries the
* PSX mask-bit mirror for the wide surface, and (the hard lesson)
* a stencil-less FBO turns every stencil-enabled mirror draw into
* ~0.6ms of driver-side work — the 16:9 GL perf collapse. */
p_glGenRenderbuffers(1, &s_wide_rb[i]);
p_glBindRenderbuffer(PSXGL_RENDERBUFFER, s_wide_rb[i]);
p_glRenderbufferStorage(PSXGL_RENDERBUFFER, PSXGL_DEPTH24_STENCIL8, w, h);
if (!make_fbo(&s_wide_fbo[i], s_wide_tex[i], s_wide_rb[i])) {
glDeleteTextures(1, &s_wide_tex[i]); s_wide_tex[i] = 0;
p_glDeleteRenderbuffers(1, &s_wide_rb[i]); s_wide_rb[i] = 0;
return 0;
}
/* Clear to black so unwritten margins are clean (not stale). */
p_glBindFramebuffer(PSXGL_FRAMEBUFFER, s_wide_fbo[i]);
glDisable(GL_SCISSOR_TEST);
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glClearStencil(0);
glStencilMask(0xFF);
glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
p_glBindFramebuffer(PSXGL_FRAMEBUFFER, 0);
s_wide_base[i] = base_x;
return s_wide_fbo[i];
@@ -2222,7 +2243,9 @@ static void glb_wide_clear(int base_x, int y, int h, uint16_t color) {
glEnable(GL_SCISSOR_TEST);
glScissor(0, y0, g_wide_w * s_scale, y1 - y0);
glClearColor(r, g, b, a);
glClear(GL_COLOR_BUFFER_BIT);
glClearStencil((color >> 15) & 1); /* stencil mirrors bit15, like the color alpha */
glStencilMask(0xFF);
glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
glDisable(GL_SCISSOR_TEST);
p_glBindFramebuffer(PSXGL_FRAMEBUFFER, 0);
gl_perf_mirror_end();