i965/fs: Constant-fold immediates in src0 of CMP instructions.

This is like what we do with add/mul, but we also have to flip the
conditional test.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/brw_eu.c

@@ -34,6 +34,28 @@
 #include "brw_defines.h"
 #include "brw_eu.h"
 
+/* Returns the corresponding conditional mod for swapping src0 and
+ * src1 in e.g. CMP.
+ */
+uint32_t
+brw_swap_cmod(uint32_t cmod)
+{
+   switch (cmod) {
+   case BRW_CONDITIONAL_Z:
+   case BRW_CONDITIONAL_NZ:
+      return cmod;
+   case BRW_CONDITIONAL_G:
+      return BRW_CONDITIONAL_LE;
+   case BRW_CONDITIONAL_GE:
+      return BRW_CONDITIONAL_L;
+   case BRW_CONDITIONAL_L:
+      return BRW_CONDITIONAL_GE;
+   case BRW_CONDITIONAL_LE:
+      return BRW_CONDITIONAL_G;
+   default:
+      return ~0;
+   }
+}
 
 
 /* How does predicate control work when execution_size != 8?  Do I

src/mesa/drivers/dri/i965/brw_eu.h

@@ -1017,6 +1017,8 @@ void brw_set_src1( struct brw_instruction *insn,
 
 void brw_set_uip_jip(struct brw_compile *p);
 
+uint32_t brw_swap_cmod(uint32_t cmod);
+
 /* brw_optimize.c */
 void brw_optimize(struct brw_compile *p);
 void brw_remove_duplicate_mrf_moves(struct brw_compile *p);

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -2938,12 +2938,33 @@ fs_visitor::propagate_constants()
 		  progress = true;
 	       }
 	       break;
+
 	    case BRW_OPCODE_CMP:
+	       if (i == 1) {
+		  scan_inst->src[i] = inst->src[0];
+		  progress = true;
+	       } else if (i == 0 && scan_inst->src[1].file != IMM) {
+		  uint32_t new_cmod;
+
+		  new_cmod = brw_swap_cmod(scan_inst->conditional_mod);
+		  if (new_cmod != ~0u) {
+		     /* Fit this constant in by swapping the operands and
+		      * flipping the test
+		      */
+		     scan_inst->src[0] = scan_inst->src[1];
+		     scan_inst->src[1] = inst->src[0];
+		     scan_inst->conditional_mod = new_cmod;
+		     progress = true;
+		  }
+	       }
+	       break;
+
 	    case BRW_OPCODE_SEL:
 	       if (i == 1) {
 		  scan_inst->src[i] = inst->src[0];
 		  progress = true;
 	       }
+	       break;
 	    }
 	 }