nak: Add a helper to reduce OpPrmt sel immediates

Only the bottom 16 bits matter of the select source matter so we can
throw away the top 16 bits and avoid any i20 encoding issues.  All of
the back-ends were already doing this except SM70 which has 32-bit
immediates anyway.  However, doing it in a common place where it's
documented is better than skattering it everywhere.  Also, doing it as
part of legalization ensures that we see the same thing in the
post-legalize IR as gets encoded.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34678>

src/nouveau/compiler/nak/ir.rs

@@ -4617,6 +4617,15 @@ impl OpPrmt {
         }
     }
 
+    /// Reduces the sel immediate, if any.
+    pub fn reduce_sel_imm(&mut self) {
+        assert!(self.sel.src_mod.is_none());
+        if let SrcRef::Imm32(sel) = &mut self.sel.src_ref {
+            // Only the bottom 16 bits matter anyway
+            *sel &= 0xffff;
+        }
+    }
+
     pub fn as_u32(&self) -> Option<u32> {
         let Some(sel) = self.get_sel() else {
             return None;

src/nouveau/compiler/nak/sm20.rs

@@ -1531,10 +1531,7 @@ impl SM20Op for OpPrmt {
         let [src0, src1] = &mut self.srcs;
         b.copy_alu_src_if_not_reg(src0, GPR, SrcType::ALU);
         b.copy_alu_src_if_not_reg(src1, GPR, SrcType::ALU);
-        if let SrcRef::Imm32(imm32) = &mut self.sel.src_ref {
-            // Only the bottom 16 bits matter anyway
-            *imm32 = *imm32 & 0xffff;
-        }
+        self.reduce_sel_imm();
     }
 
     fn encode(&self, e: &mut SM20Encoder<'_>) {

src/nouveau/compiler/nak/sm32.rs

@@ -1778,11 +1778,7 @@ impl SM32Op for OpPrmt {
         use RegFile::GPR;
         b.copy_alu_src_if_not_reg(&mut self.srcs[0], GPR, SrcType::GPR);
         b.copy_alu_src_if_not_reg(&mut self.srcs[1], GPR, SrcType::GPR);
-
-        if let SrcRef::Imm32(imm32) = &mut self.sel.src_ref {
-            // Only the bottom 16 bits matter anyway
-            *imm32 = *imm32 & 0xffff;
-        }
+        self.reduce_sel_imm();
     }
 
     fn encode(&self, e: &mut SM32Encoder<'_>) {

src/nouveau/compiler/nak/sm50.rs

@@ -1959,6 +1959,7 @@ impl SM50Op for OpPrmt {
         use RegFile::GPR;
         b.copy_alu_src_if_not_reg(&mut self.srcs[0], GPR, SrcType::GPR);
         b.copy_alu_src_if_not_reg(&mut self.srcs[1], GPR, SrcType::GPR);
+        self.reduce_sel_imm();
     }
 
     fn encode(&self, e: &mut SM50Encoder<'_>) {
@@ -1970,7 +1971,7 @@ impl SM50Op for OpPrmt {
             SrcRef::Imm32(imm32) => {
                 e.set_opcode(0x36c0);
                 // Only the bottom 16 bits matter
-                e.set_src_imm_i20(20..39, 56, *imm32 & 0xffff);
+                e.set_src_imm_i20(20..39, 56, *imm32);
             }
             SrcRef::CBuf(cb) => {
                 e.set_opcode(0x4bc0);

src/nouveau/compiler/nak/sm70_encode.rs

@@ -2043,6 +2043,7 @@ impl SM70Op for OpPrmt {
         let [src0, src1] = &mut self.srcs;
         b.copy_alu_src_if_not_reg(src0, gpr, SrcType::ALU);
         b.copy_alu_src_if_not_reg(src1, gpr, SrcType::ALU);
+        self.reduce_sel_imm();
     }
 
     fn encode(&self, e: &mut SM70Encoder<'_>) {