pan/midgard: Document branch combination LUT

This took way longer to figure out than it should have..

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

src/panfrost/midgard/disassemble.c

@@ -740,15 +740,18 @@ print_extended_branch_writeout_field(uint8_t *words)
 
         print_branch_op(br.op);
 
-        /* Condition repeated 8 times in all known cases. Check this. */
+        /* Condition codes are a LUT in the general case, but simply repeated 8 times for single-channel conditions.. Check this. */
 
-        unsigned cond = br.cond & 0x3;
+        bool single_channel = true;
 
         for (unsigned i = 0; i < 16; i += 2) {
-                assert(((br.cond >> i) & 0x3) == cond);
+                single_channel &= (((br.cond >> i) & 0x3) == (br.cond & 0x3));
         }
 
-        print_branch_cond(cond);
+        if (single_channel)
+                print_branch_cond(br.cond & 0x3);
+        else
+                printf("lut%X", br.cond);
 
         if (br.unknown)
                 printf(".unknown%d", br.unknown);

src/panfrost/midgard/midgard.h

@@ -359,6 +359,19 @@ __attribute__((__packed__))
         unsigned dest_tag : 4; /* tag of branch destination */
         unsigned unknown : 2;
         signed offset : 23;
+
+        /* Extended branches permit inputting up to 4 conditions loaded into
+         * r31 (two in r31.w and two in r31.x). In the most general case, we
+         * specify a function f(A, B, C, D) mapping 4 1-bit conditions to a
+         * single 1-bit branch criteria. Note that the domain of f has 2^(2^4)
+         * elements, each mapping to 1-bit of output, so we can trivially
+         * construct a Godel numbering of f as a (2^4)=16-bit integer. This
+         * 16-bit integer serves as a lookup table to compute f, subject to
+         * some swaps for ordering.
+         *
+         * Interesting, the standard 2-bit condition codes are also a LUT with
+         * the same format (2^1-bit), but it's usually easier to use enums. */
+
         unsigned cond : 16;
 }
 midgard_branch_extended;

src/panfrost/midgard/midgard_compile.c

@@ -224,7 +224,11 @@ midgard_create_branch_extended( midgard_condition cond,
                                 unsigned dest_tag,
                                 signed quadword_offset)
 {
-        /* For unclear reasons, the condition code is repeated 8 times */
+        /* The condition code is actually a LUT describing a function to
+         * combine multiple condition codes. However, we only support a single
+         * condition code at the moment, so we just duplicate over a bunch of
+         * times. */
+
         uint16_t duplicated_cond =
                 (cond << 14) |
                 (cond << 12) |