nak: Add ShaderModel::hw_reserved_gprs()

Cc: mesa-stable
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
(cherry picked from commit 914c722eb0)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33113>

.pick_status.json

@@ -1684,7 +1684,7 @@
         "description": "nak: Add ShaderModel::hw_reserved_gprs()",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/nouveau/compiler/nak/api.rs

@@ -231,10 +231,10 @@ impl ShaderBin {
                 ShaderStageInfo::Tessellation(_) => MESA_SHADER_TESS_EVAL,
             },
             sm: sm.sm(),
-            num_gprs: if sm.sm() >= 70 {
-                max(4, info.num_gprs + 2)
-            } else {
-                max(4, info.num_gprs)
+            num_gprs: {
+                max(4, info.num_gprs as u32 + sm.hw_reserved_gprs())
+                    .try_into()
+                    .unwrap()
             },
             num_control_barriers: info.num_control_barriers,
             _pad0: Default::default(),

src/nouveau/compiler/nak/ir.rs

@@ -7250,6 +7250,7 @@ pub struct ShaderInfo {
 pub trait ShaderModel {
     fn sm(&self) -> u8;
     fn num_regs(&self, file: RegFile) -> u32;
+    fn hw_reserved_gprs(&self) -> u32;
     fn crs_size(&self, max_crs_depth: u32) -> u32;
 
     fn op_can_be_uniform(&self, op: &Op) -> bool;

src/nouveau/compiler/nak/sm50.rs

@@ -38,6 +38,10 @@ impl ShaderModel for ShaderModel50 {
         }
     }
 
+    fn hw_reserved_gprs(&self) -> u32 {
+        0
+    }
+
     fn crs_size(&self, max_crs_depth: u32) -> u32 {
         if max_crs_depth <= 16 {
             0

src/nouveau/compiler/nak/sm70.rs

@@ -33,12 +33,7 @@ impl ShaderModel for ShaderModel70 {
 
     fn num_regs(&self, file: RegFile) -> u32 {
         match file {
-            RegFile::GPR => {
-                // Volta+ has a maximum of 253 registers.  Presumably
-                // because two registers get burned for UGPRs? Unclear
-                // on why we need it on Volta though.
-                253
-            }
+            RegFile::GPR => 255 - self.hw_reserved_gprs(),
             RegFile::UGPR => {
                 if self.has_uniform_alu() {
                     63
@@ -60,6 +55,13 @@ impl ShaderModel for ShaderModel70 {
         }
     }
 
+    fn hw_reserved_gprs(&self) -> u32 {
+        // On Volta+, 2 GPRs get burned for the program counter - see the
+        // footnote on table 2 of the volta whitepaper
+        // https://images.nvidia.com/content/volta-architecture/pdf/volta-architecture-whitepaper.pdf
+        2
+    }
+
     fn crs_size(&self, max_crs_depth: u32) -> u32 {
         assert!(max_crs_depth == 0);
         0