nir: Add an "exact" bit to nir_alu_instr

Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/compiler/nir/nir.h

@@ -718,6 +718,17 @@ extern const nir_op_info nir_op_infos[nir_num_opcodes];
 typedef struct nir_alu_instr {
    nir_instr instr;
    nir_op op;
+
+   /** Indicates that this ALU instruction generates an exact value
+    *
+    * This is kind of a mixture of GLSL "precise" and "invariant" and not
+    * really equivalent to either.  This indicates that the value generated by
+    * this operation is high-precision and any code transformations that touch
+    * it must ensure that the resulting value is bit-for-bit identical to the
+    * original.
+    */
+   bool exact;
+
    nir_alu_dest dest;
    nir_alu_src src[];
 } nir_alu_instr;

src/compiler/nir/nir_clone.c

@@ -312,6 +312,7 @@ static nir_alu_instr *
 clone_alu(clone_state *state, const nir_alu_instr *alu)
 {
    nir_alu_instr *nalu = nir_alu_instr_create(state->ns, alu->op);
+   nalu->exact = alu->exact;
 
    __clone_dst(state, &nalu->instr, &nalu->dest.dest, &alu->dest.dest);
    nalu->dest.saturate = alu->dest.saturate;

src/compiler/nir/nir_print.c

@@ -207,6 +207,8 @@ print_alu_instr(nir_alu_instr *instr, print_state *state)
    print_alu_dest(&instr->dest, state);
 
    fprintf(fp, " = %s", nir_op_infos[instr->op].name);
+   if (instr->exact)
+      fprintf(fp, "!");
    if (instr->dest.saturate)
       fprintf(fp, ".sat");
    fprintf(fp, " ");