Mesa (staging/21.3): pan/bi: Avoid *FADD.v2f16 hazard in scheduler

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Module: Mesa
Branch: staging/21.3
Commit: e7222259a3b722e4d5403e1652403a6111e00b7c
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=e7222259a3b722e4d5403e1652403a6111e00b7c

Author: Alyssa Rosenzweig <alyssa at collabora.com>
Date:   Thu Feb 17 19:34:04 2022 -0500

pan/bi: Avoid *FADD.v2f16 hazard in scheduler

Obscure encoding restriction. Fixes crash (assertion fail when instruction
packing) in asphalt9/2659.shader_test on Bifrost.

Signed-off-by: Alyssa Rosenzweig <alyssa at collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15072>
(cherry picked from commit 24d2bdb1e050134a25924487792ee0018f8478ae)

---

 .pick_status.json                  |  2 +-
 src/panfrost/bifrost/bi_schedule.c | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/.pick_status.json b/.pick_status.json
index 3a4da06c144..a66aed88978 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -481,7 +481,7 @@
         "description": "pan/bi: Avoid *FADD.v2f16 hazard in scheduler",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null
     },
diff --git a/src/panfrost/bifrost/bi_schedule.c b/src/panfrost/bifrost/bi_schedule.c
index cc0c128f1ea..421afdffc40 100644
--- a/src/panfrost/bifrost/bi_schedule.c
+++ b/src/panfrost/bifrost/bi_schedule.c
@@ -490,6 +490,18 @@ bi_can_iaddc(bi_instr *ins)
                 ins->src[1].swizzle == BI_SWIZZLE_H01);
 }
 
+/*
+ * The encoding of *FADD.v2f16 only specifies a single abs flag. All abs
+ * encodings are permitted by swapping operands; however, this scheme fails if
+ * both operands are equal. Test for this case.
+ */
+static bool
+bi_impacted_abs(bi_instr *I)
+{
+        return I->src[0].abs && I->src[1].abs &&
+               bi_is_word_equiv(I->src[0], I->src[1]);
+}
+
 bool
 bi_can_fma(bi_instr *ins)
 {
@@ -497,6 +509,10 @@ bi_can_fma(bi_instr *ins)
         if (bi_can_iaddc(ins))
                 return true;
 
+        /* *FADD.v2f16 has restricted abs modifiers, use +FADD.v2f16 instead */
+        if (ins->op == BI_OPCODE_FADD_V2F16 && bi_impacted_abs(ins))
+                return false;
+
         /* TODO: some additional fp16 constraints */
         return bi_opcode_props[ins->op].fma;
 }