nak: Add a debug flag to test spilling

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

src/nouveau/compiler/nak.rs

@@ -40,6 +40,7 @@ use util::NextMultipleOf;
 enum DebugFlags {
     Print,
     Serial,
+    Spill,
 }
 
 struct Debug {
@@ -61,6 +62,7 @@ impl Debug {
             match flag.trim() {
                 "print" => flags |= 1 << DebugFlags::Print as u8,
                 "serial" => flags |= 1 << DebugFlags::Serial as u8,
+                "spill" => flags |= 1 << DebugFlags::Spill as u8,
                 unk => eprintln!("Unknown NAK_DEBUG flag \"{}\"", unk),
             }
         }
@@ -78,6 +80,10 @@ trait GetDebugFlags {
     fn serial(&self) -> bool {
         self.debug_flags() & (1 << DebugFlags::Serial as u8) != 0
     }
+
+    fn spill(&self) -> bool {
+        self.debug_flags() & (1 << DebugFlags::Spill as u8) != 0
+    }
 }
 
 static DEBUG: OnceLock<Debug> = OnceLock::new();

src/nouveau/compiler/nak_ir.rs

@@ -9,6 +9,7 @@ pub use crate::nak_builder::{
     Builder, InstrBuilder, SSABuilder, SSAInstrBuilder,
 };
 use crate::nak_cfg::CFG;
+use crate::{GetDebugFlags, DEBUG};
 use nak_ir_proc::*;
 use std::fmt;
 use std::iter::Zip;
@@ -77,7 +78,11 @@ impl RegFile {
     pub fn num_regs(&self, sm: u8) -> u32 {
         match self {
             RegFile::GPR => {
-                if sm >= 75 {
+                if DEBUG.spill() {
+                    // We need at least 16 registers to satisfy RA constraints
+                    // for texture ops and another 2 for parallel copy lowering
+                    18
+                } else if sm >= 75 {
                     // Turing+ has a maximum of 253 registers.  Presumably
                     // because two registers get burned for UGPRs?
                     253