[PATCH][AArch64] Allow CMP+SHIFT when comparing with zero

[Sudi Das <Sudi.Das@arm.com>]

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Hi all

The backend pattern for combining a CMP+SHIFT was missing out on a case when comparing with zero. This was happening because aarch64_select_cc_mode (SELECT_CC_MODE) was not returning the correct mode (in this case CC_SWP) which was needed to identify the combine. This patch adds this missing case.

For the test case :


int f3 (int x, int y)
{
  int res = x << 3;
  return res != 0;
}

We are now generating (at -O2)

f3:
        cmp     wzr, w0, lsl 3
        cset    w0, ne
        ret

instead of :

f3:
        lsl     w0, w0, 3
        cmp     w0, 0
        cset    w0, ne
        ret


Added this new test and checked for regressions on bootstrapped aarch64-none-linux-gnu.
Ok for stage 1?

Thanks
Sudi


2017-03-10  Sudakshina Das  <sudi.das@arm.com>

	* config/aarch64/aarch64.c (aarch64_select_cc_mode): Return CC_SWP for
	comparision with zero.
    
2017-03-10  Sudakshina Das  <sudi.das@arm.com>

	* gcc.target/aarch64/cmp_shifted_reg_1.c: New Test.

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diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 714bb79..01af2a7 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -4707,7 +4707,7 @@ aarch64_select_cc_mode (RTX_CODE code, rtx x, rtx y)
      the comparison will have to be swapped when we emit the assembly
      code.  */
   if ((GET_MODE (x) == SImode || GET_MODE (x) == DImode)
-      && (REG_P (y) || GET_CODE (y) == SUBREG)
+      && (REG_P (y) || GET_CODE (y) == SUBREG || y == const0_rtx)
       && (GET_CODE (x) == ASHIFT || GET_CODE (x) == ASHIFTRT
 	  || GET_CODE (x) == LSHIFTRT
 	  || GET_CODE (x) == ZERO_EXTEND || GET_CODE (x) == SIGN_EXTEND))
diff --git a/gcc/testsuite/gcc.target/aarch64/cmp_shifted_reg_1.c b/gcc/testsuite/gcc.target/aarch64/cmp_shifted_reg_1.c
new file mode 100644
index 0000000..cacecf4
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/cmp_shifted_reg_1.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 " } */
+
+int f3 (int x, int y)
+{
+  int res = x << 3;
+  return res != 0;
+}
+
+/* We should combine the shift and compare */
+/* { dg-final { scan-assembler "cmp\.*\twzr, w\[0-9\]+, lsl 3" } } */