From c0efa612cb377692a782e0d1879881433e73da62 Mon Sep 17 00:00:00 2001
From: Dave Airlie <airlied@redhat.com>
Date: Thu, 27 Nov 2025 10:30:28 +1000
Subject: [PATCH] nir/algebraic: optimise ishl + ubfe into iand + ishl/ushr

I've no idea if this correct, but I've got an insane shader that

-Spilled VGPRs: 4063
-Code size: 253708
+Spilled VGPRs: 3635
+Code size: 226124
-Instructions: 41025
-Copies: 4
+Instructions: 40112
+Copies: 1028
---
 src/compiler/nir/nir_opt_algebraic.py | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 10b2773d8c5..588d76ebdcd 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -594,6 +594,14 @@ def ubfe_ubfe(a, b, c, d, e):
     # whose offset and bits operands will also be constant folded.
     return ('bcsel', offset_out_of_range, 0, collapsed)
 
+# drop ubfe + ushl - into iand + ushl/ushr
+def ubfe_iand(a, b, c, d):
+    iand_val = ('iand', a, ('ishl', ('isub', ('ishl', 1, c), 1), b))
+    ishl_val = ('ishl', iand_val, ('isub', d, b))
+    ushr_val = ('ushr', iand_val, ('isub', b, d))
+    cmp_val = ('ilt', b, d)
+    return ('bcsel', cmp_val, ishl_val, ushr_val)
+
 optimizations.extend([
     # Create bitfield extract from right-shift + and pattern.
     (('iand@32', ('ushr@32(is_used_once)', a, b), '#c(is_const_bitmask)'),
@@ -620,6 +628,10 @@ optimizations.extend([
     (('ushr@32', ('ubfe', a, '#b', '#c'), '#d(is_5lsb_not_zero)'),
      ubfe_ubfe(a, b, c, d, 31)),
 
+    # Replace ishl/ubfe with ishl/iand
+    (('ishl', ('ubfe', a, '#b', '#c'), '#d'),
+     ubfe_iand(a, b, c, d)),
+
     (('iand', 'a(is_unsigned_multiple_of_4)', -4), a),
 ])
 
-- 
2.52.0