Re: [PATCH: PR target/46975] Replace 32 bit instructions with 16 bit instructions in thumb2

[Carrot Wei <carrot at google dot com>]

On Fri, Dec 17, 2010 at 8:18 PM, Richard Earnshaw <rearnsha@arm.com> wrote:
>
> On Thu, 2010-12-16 at 14:45 -0800, Carrot Wei wrote:
>> Hi
>>
>> Compile the following c code with options -march=armv7-a -mthumb -Os
>>
>> int foo (int s)
>> {
>> ? ? return s == 1;
>> }
>>
>> GCC 4.6 generates:
>>
>> 00000000 <foo>:
>> ? ?0: ? ?f1a0 0301 ? ? sub.w ? ?r3, r0, #1 ? ?// A
>> ? ?4: ? ?4258 ? ? ? ? ?negs ? ?r0, r3
>> ? ?6: ? ?eb40 0003 ? ? adc.w ? ?r0, r0, r3 ? ? ?// B
>> ? ?a: ? ?4770 ? ? ? ? ?bx ? ?lr
>>
>> Notice that instructions A and B are 32 bits. In thumb2 we can use subs and adcs
>> instead so they will be 16 bits.
>>
>
> This sequence already contains an instruction that sets the flags as a
> necessary part of the sequence. ?Why doesn't it also generate
> flag-corrupting variants of the other two instructions when the
> registers selected are suitable? ?It seems silly to force the compiler
> to do yet more work to clean up this code.
>

This revised patch uses new adc insn which simply clobber the CC
register. For the sub instruction I still use the CC setting form
since the existing pattern subsi3_compare does this.

Tested on arm qemu without regression.

thanks
Carrot

ChangeLog:
2011-05-16 ?Wei Guozhi ?<carrot@google.com>

? ? ? ? PR target/46975
 ? ? ? ?* config/arm/arm.md (*addsi3_carryin_compare0_<optab>): New pattern.
? ? ? ? (peephole2 for conditional move): Generate 16 bit instructions.

ChangeLog:
2010-05-16 ?Wei Guozhi ?<carrot@google.com>

 ? ? ? ?PR target/46975
 ? ? ? ?* gcc.target/arm/pr46975.c: New testcase.

Index: testsuite/gcc.target/arm/pr46975.c
===================================================================
--- testsuite/gcc.target/arm/pr46975.c	(revision 0)
+++ testsuite/gcc.target/arm/pr46975.c	(revision 0)
@@ -0,0 +1,9 @@
+/* { dg-options "-mthumb -Os" } */
+/* { dg-require-effective-target arm_thumb2_ok } */
+/* { dg-final { scan-assembler "subs" } } */
+/* { dg-final { scan-assembler "adcs" } } */
+
+int foo (int s)
+{
+      return s == 1;
+}
Index: config/arm/arm.md
===================================================================
--- config/arm/arm.md	(revision 173770)
+++ config/arm/arm.md	(working copy)
@@ -985,6 +985,17 @@
 		      (const_string "alu_shift_reg")))]
 )

+(define_insn "*addsi3_carryin_clobercc_<optab>"
+  [(set (match_operand:SI 0 "s_register_operand" "=r")
+	(plus:SI (plus:SI (match_operand:SI 1 "s_register_operand" "%r")
+			  (match_operand:SI 2 "arm_rhs_operand" "rI"))
+		 (LTUGEU:SI (reg:<cnb> CC_REGNUM) (const_int 0))))
+   (clobber (reg:CC CC_REGNUM))]
+   "TARGET_32BIT"
+   "adc%.\\t%0, %1, %2"
+   [(set_attr "conds" "set")]
+)
+
 (define_expand "incscc"
   [(set (match_operand:SI 0 "s_register_operand" "=r,r")
         (plus:SI (match_operator:SI 2 "arm_comparison_operator"
@@ -8788,14 +8799,19 @@
 	      (set (match_dup 0) (const_int 1)))
    (match_scratch:SI 3 "r")]
   "TARGET_32BIT"
-  [(set (match_dup 3) (minus:SI (match_dup 1) (match_dup 2)))
+  [(parallel
+    [(set (reg:CC CC_REGNUM)
+	  (compare:CC (match_dup 1) (match_dup 2)))
+     (set (match_dup 3) (minus:SI (match_dup 1) (match_dup 2)))])
    (parallel
     [(set (reg:CC CC_REGNUM)
 	  (compare:CC (const_int 0) (match_dup 3)))
      (set (match_dup 0) (minus:SI (const_int 0) (match_dup 3)))])
-   (set (match_dup 0)
-	(plus:SI (plus:SI (match_dup 0) (match_dup 3))
-		 (geu:SI (reg:CC CC_REGNUM) (const_int 0))))])
+   (parallel
+    [(set (match_dup 0)
+	  (plus:SI (plus:SI (match_dup 0) (match_dup 3))
+		   (geu:SI (reg:CC CC_REGNUM) (const_int 0))))
+     (clobber (reg:CC CC_REGNUM))])])

 (define_insn "*cond_move"
   [(set (match_operand:SI 0 "s_register_operand" "=r,r,r")