[PATCH v2] Fold (add -1; zero_ext; add +1) operations to zero_ext when not overflow (PR37451, part of PR61837)

[luoxhu]

Minor refine of checking iterations nonoverflow and a testcase for stage 1.


This "subtract/extend/add" existed for a long time and still annoying us
(PR37451, part of PR61837) when converting from 32bits to 64bits, as the ctr
register is used as 64bits on powerpc64, Andraw Pinski had a patch but
caused some issue and reverted by Joseph S. Myers(PR37451, PR37782).

Andraw:
http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01070.html
http://gcc.gnu.org/ml/gcc-patches/2008-10/msg01321.html
Joseph:
https://gcc.gnu.org/legacy-ml/gcc-patches/2011-11/msg02405.html

We can do the simplification from "subtract/extend/add" to only extend
when loop iterations is known to be LT than MODE_MAX-1(NOT do simplify
when counter+0x1 overflow).

Bootstrap and regression tested pass on Power8-LE.

gcc/ChangeLog

	2020-05-12  Xiong Hu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/37451, part of PR target/61837
	* loop-doloop.c (doloop_modify): Simplify (add -1; zero_ext; add +1)
	to zero_ext when not wrapping overflow.

gcc/testsuite/ChangeLog

	2020-05-12  Xiong Hu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/37451, part of PR target/61837
	* gcc.target/powerpc/doloop-2.c: New test.
---
 gcc/loop-doloop.c                           | 46 ++++++++++++++++++++-
 gcc/testsuite/gcc.target/powerpc/doloop-2.c | 14 +++++++
 2 files changed, 59 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/doloop-2.c

diff --git a/gcc/loop-doloop.c b/gcc/loop-doloop.c
index db6a014e43d..16372382a22 100644
--- a/gcc/loop-doloop.c
+++ b/gcc/loop-doloop.c
@@ -477,7 +477,51 @@ doloop_modify (class loop *loop, class niter_desc *desc,
     }
 
   if (increment_count)
-    count = simplify_gen_binary (PLUS, mode, count, const1_rtx);
+    {
+      /* Fold (add -1; zero_ext; add +1) operations to zero_ext. i.e:
+
+	 73: r145:SI=r123:DI#0-0x1
+	 74: r144:DI=zero_extend (r145:SI)
+	 75: r143:DI=r144:DI+0x1
+	 ...
+	 31: r135:CC=cmp (r123:DI,0)
+	 72: {pc={(r143:DI!=0x1)?L70:pc};r143:DI=r143:DI-0x1;clobber
+	 scratch;clobber scratch;}
+
+	 r123:DI#0-0x1 is param count derived from loop->niter_expr equal to the
+	 loop iterations, if loop iterations expression doesn't overflow, then
+	 (zero_extend (r123:DI#0-1))+1 could be simplified to zero_extend only.
+       */
+      bool simplify_zext = false;
+      rtx extop0 = XEXP (count, 0);
+      if (GET_CODE (count) == ZERO_EXTEND && GET_CODE (extop0) == PLUS)
+	{
+	  rtx addop0 = XEXP (extop0, 0);
+	  rtx addop1 = XEXP (extop0, 1);
+
+	  int nonoverflow = 0;
+	  unsigned int_mode
+	    = GET_MODE_PRECISION (as_a<scalar_int_mode> GET_MODE (addop0));
+	  unsigned HOST_WIDE_INT int_mode_max
+	    = (HOST_WIDE_INT_1U << (int_mode - 1) << 1) - 1;
+	  if (get_max_loop_iterations (loop, &iterations)
+	      && wi::ltu_p (iterations, int_mode_max))
+	    nonoverflow = 1;
+
+	  if (nonoverflow
+	      && CONST_SCALAR_INT_P (addop1)
+	      && GET_MODE_PRECISION (mode) == int_mode * 2
+	      && addop1 == GEN_INT (-1))
+	    {
+	      count = simplify_gen_unary (ZERO_EXTEND, mode, addop0,
+					  GET_MODE (addop0));
+	      simplify_zext = true;
+	    }
+	}
+
+      if (!simplify_zext)
+	count = simplify_gen_binary (PLUS, mode, count, const1_rtx);
+    }
 
   /* Insert initialization of the count register into the loop header.  */
   start_sequence ();
diff --git a/gcc/testsuite/gcc.target/powerpc/doloop-2.c b/gcc/testsuite/gcc.target/powerpc/doloop-2.c
new file mode 100644
index 00000000000..dc8516bb0ab
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/doloop-2.c
@@ -0,0 +1,14 @@
+/* { dg-do compile { target powerpc*-*-* } } */
+/* { dg-options "-O2 -fno-unroll-loops" } */
+
+int f(int l, int *a)
+{
+    int i;
+      for(i = 0;i < l; i++)
+	    a[i] = i;
+        return l;
+}
+
+/* { dg-final { scan-assembler-not "-1" } } */
+/* { dg-final { scan-assembler "bdnz" } } */
+/* { dg-final { scan-assembler-times "mtctr" 1 } } */
-- 
2.21.0.777.g83232e3864