Mesa (main): broadcom/compiler: disable flags optimization for loop conditions

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Module: Mesa
Branch: main
Commit: a97f78eb144ebc8964b891bbf708659e70e5f1c3
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=a97f78eb144ebc8964b891bbf708659e70e5f1c3

Author: Iago Toral Quiroga <itoral at igalia.com>
Date:   Fri Jun 10 10:46:51 2022 +0200

broadcom/compiler: disable flags optimization for loop conditions

This is not safe because it may skip regenerating the flags for the
loop condition in the loop continue block and these flags may be
stomped in the loop body by other conditionals.

Fixes: 9909fe6ba ('broadcom/compiler: Skip bool_to_cond where possible')
Reviewed-by: Alejandro Piñeiro <apinheiro at igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17020>

---

 src/broadcom/compiler/nir_to_vir.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/broadcom/compiler/nir_to_vir.c b/src/broadcom/compiler/nir_to_vir.c
index 852c454ffdf..2bdbccad308 100644
--- a/src/broadcom/compiler/nir_to_vir.c
+++ b/src/broadcom/compiler/nir_to_vir.c
@@ -4170,7 +4170,6 @@ ntq_emit_nonuniform_loop(struct v3d_compile *c, nir_loop *loop)
 static void
 ntq_emit_uniform_loop(struct v3d_compile *c, nir_loop *loop)
 {
-
         c->loop_cont_block = vir_new_block(c);
         c->loop_break_block = vir_new_block(c);
 
@@ -4190,6 +4189,23 @@ ntq_emit_uniform_loop(struct v3d_compile *c, nir_loop *loop)
 static void
 ntq_emit_loop(struct v3d_compile *c, nir_loop *loop)
 {
+        /* Disable flags optimization for loop conditions. The problem here is
+         * that we can have code like this:
+         *
+         *  // block_0
+         *  vec1 32 con ssa_9 = ine32 ssa_8, ssa_2
+         *  loop {
+         *     // block_1
+         *     if ssa_9 {
+         *
+         * In this example we emit flags to compute ssa_9 and the optimization
+         * will skip regenerating them again for the loop condition in the
+         * loop continue block (block_1). However, this is not safe after the
+         * first iteration because the loop body can stomp the flags if it has
+         * any conditionals.
+         */
+        c->flags_temp = -1;
+
         bool was_in_control_flow = c->in_control_flow;
         c->in_control_flow = true;