[patch, ARM] Add mulhi3 expander.

[patch, ARM] Add mulhi3 expander.

[Richard Earnshaw]

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When we have the DSP multiply extension (armv5e and up for ARM or T2), 
then a 16-bit multiply is best done with smulbb rather than mul.  This 
saves us doing redundant extends and may even be a faster instruction on 
some cores.  Unfortunately, expand does not try this operation by 
default; but it's trivial to add to the back-end.  Since the value 
produced is extended, we represent this by expanding into the widening 
mul version and then subreg-ing the result.

	* arm.md (mulhi3): New expand pattern.

R.

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--- gcc/config/arm/arm.md	(revision 201327)
+++ gcc/config/arm/arm.md	(local)
@@ -1725,6 +1725,20 @@ (define_expand "subdf3"
 \f
 ;; Multiplication insns
 
+(define_expand "mulhi3"
+  [(set (match_operand:HI 0 "s_register_operand" "")
+	(mult:HI (match_operand:HI 1 "s_register_operand" "")
+		 (match_operand:HI 2 "s_register_operand" "")))]
+  "TARGET_DSP_MULTIPLY"
+  "
+  {
+    rtx result = gen_reg_rtx (SImode);
+    emit_insn (gen_mulhisi3 (result, operands[1], operands[2]));
+    emit_move_insn (operands[0], gen_lowpart (HImode, result));
+    DONE;
+  }"
+)
+
 (define_expand "mulsi3"
   [(set (match_operand:SI          0 "s_register_operand" "")
 	(mult:SI (match_operand:SI 2 "s_register_operand" "")