[PATCH] Fold truncations of left shifts in match.pd

["Roger Sayle" <roger@nextmovesoftware.com>]

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Whilst investigating PR 55278, I noticed that the tree-ssa optimizers
aren't eliminating the promotions of shifts to "int" as inserted by the
c-family front-ends, instead leaving this simplification to be left to
the RTL optimizers.  This patch allows match.pd to do this itself earlier,
narrowing (T)(X << C) to (T)X << C when the constant C is known to be
valid for the (narrower) type T.

Hence for this simple test case:
short foo(short x) { return x << 5; }

the .optimized dump currently looks like:

short int foo (short int x)
{
  int _1;
  int _2;
  short int _4;

  <bb 2> [local count: 1073741824]:
  _1 = (int) x_3(D);
  _2 = _1 << 5;
  _4 = (short int) _2;
  return _4;
}

but with this patch, now becomes:

short int foo (short int x)
{
  short int _2;

  <bb 2> [local count: 1073741824]:
  _2 = x_1(D) << 5;
  return _2;
}

This is always reasonable as RTL expansion knows how to use
widening optabs if it makes sense at the RTL level to perform
this shift in a wider mode.

Of course, there's often a catch.  The above simplification not only
reduces the number of statements in gimple, but also allows further
optimizations, for example including the perception of rotate idioms
and bswap16.  Alas, optimizing things earlier than anticipated
requires several testsuite changes [though all these tests have
been confirmed to generate identical assembly code on x86_64].
The only significant change is that the vectorization pass previously
wouldn't vectorize rotations if the backend doesn't explicitly provide
an optab for them.  This is curious as if the rotate is expressed as
ior(lshift,rshift) it will vectorize, and likewise RTL expansion will
generate the iorv(lshiftv,rshiftv) sequence if required for a vector
mode rotation.  Hence this patch includes a tweak to the optabs
test in tree-vect-stmts.cc's vectorizable_shifts to better reflect
the functionality supported by RTL expansion.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures.  Ok for mainline?


2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * match.pd (convert (lshift @1 INTEGER_CST@2)): Narrow integer
        left shifts by a constant when the result is truncated, and the
        shift constant is well-defined for the narrower mode.
        * tree-vect-stmts.cc (vectorizable_shift): Rotations by
        constants are vectorizable, if the backend supports logical
        shifts and IOR logical operations in the required vector mode.

gcc/testsuite/ChangeLog
        * gcc.dg/fold-convlshift-4.c: New test case.
        * gcc.dg/optimize-bswaphi-1.c: Update found bswap count.
        * gcc.dg/tree-ssa/pr61839_3.c: Shift is now optimized before VRP.
        * gcc.dg/vect/vect-over-widen-1-big-array.c: Remove obsolete tests.
        * gcc.dg/vect/vect-over-widen-1.c: Likewise.
        * gcc.dg/vect/vect-over-widen-3-big-array.c: Likewise.
        * gcc.dg/vect/vect-over-widen-3.c: Likewise.
        * gcc.dg/vect/vect-over-widen-4-big-array.c: Likewise.
        * gcc.dg/vect/vect-over-widen-4.c: Likewise.


Thanks in advance,
Roger
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diff --git a/gcc/match.pd b/gcc/match.pd
index c2fed9b..05b34d6 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3637,6 +3637,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
       && !integer_zerop (@3))
   (lshift (convert @2) @3)))
 
+/* Narrow a lshift by constant.  */
+(simplify
+ (convert (lshift:s@0 @1 INTEGER_CST@2))
+ (if (INTEGRAL_TYPE_P (type)
+      && INTEGRAL_TYPE_P (TREE_TYPE (@0))
+      && TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@0))
+      && !integer_zerop (@2)
+      && wi::ltu_p (wi::to_wide (@2), TYPE_PRECISION (type)))
+  (lshift (convert @1) (convert @2))))
+
 /* Simplifications of conversions.  */
 
 /* Basic strip-useless-type-conversions / strip_nops.  */
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-4.c b/gcc/testsuite/gcc.dg/fold-convlshift-4.c
new file mode 100644
index 0000000..001627f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-4.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+short foo(short x)
+{
+  return x << 5;
+}
+
+/* { dg-final { scan-tree-dump-not "\\(int\\)" "optimized" } } */
+/* { dg-final { scan-tree-dump-not "\\(short int\\)" "optimized" } } */
diff --git a/gcc/testsuite/gcc.dg/optimize-bswaphi-1.c b/gcc/testsuite/gcc.dg/optimize-bswaphi-1.c
index d045da9..a5d8bfd 100644
--- a/gcc/testsuite/gcc.dg/optimize-bswaphi-1.c
+++ b/gcc/testsuite/gcc.dg/optimize-bswaphi-1.c
@@ -68,4 +68,4 @@ get_unaligned_16_be (unsigned char *p)
 
 
 /* { dg-final { scan-tree-dump-times "16 bit load in target endianness found at" 4 "bswap" } } */
-/* { dg-final { scan-tree-dump-times "16 bit bswap implementation found at" 5 "bswap" } } */
+/* { dg-final { scan-tree-dump-times "16 bit bswap implementation found at" 4 "bswap" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr61839_3.c b/gcc/testsuite/gcc.dg/tree-ssa/pr61839_3.c
index bc2126f..38cf792 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr61839_3.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr61839_3.c
@@ -1,6 +1,6 @@
 /* PR tree-optimization/61839.  */
 /* { dg-do run } */
-/* { dg-options "-O2 -fdump-tree-vrp -fdump-tree-optimized -fdisable-tree-ethread -fdisable-tree-threadfull1" } */
+/* { dg-options "-O2 -fdump-tree-optimized -fdisable-tree-ethread -fdisable-tree-threadfull1" } */
 
 __attribute__ ((noinline))
 int foo (int a, unsigned b)
@@ -21,6 +21,4 @@ int main ()
   foo (-1, b);
 }
 
-/* Scan for c [12, 13] << 8 in function foo.  */
-/* { dg-final { scan-tree-dump-times "3072 : 3328" 1  "vrp1" } } */
 /* { dg-final { scan-tree-dump-times "3072" 0  "optimized" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-1-big-array.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-1-big-array.c
index 9e5f464..9a5141ee 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-1-big-array.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-1-big-array.c
@@ -58,9 +58,7 @@ int main (void)
 }
 
 /* { dg-final { scan-tree-dump-times "vect_recog_widen_shift_pattern: detected" 2 "vect" { target vect_widen_shift } } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 8} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 5} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-1.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-1.c
index c2d0797..f2d284c 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-1.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-1.c
@@ -62,9 +62,7 @@ int main (void)
 }
 
 /* { dg-final { scan-tree-dump-times "vect_recog_widen_shift_pattern: detected" 2 "vect" { target vect_widen_shift } } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 8} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 5} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-3-big-array.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-3-big-array.c
index 37da7c9..6f89aac 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-3-big-array.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-3-big-array.c
@@ -59,9 +59,7 @@ int main (void)
   return 0;
 }
 
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 8} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 9} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-3.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-3.c
index 4138480..a1e1182 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-3.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-3.c
@@ -57,9 +57,7 @@ int main (void)
   return 0;
 }
 
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 8} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 9} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-4-big-array.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-4-big-array.c
index 514337c..03a6e67 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-4-big-array.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-4-big-array.c
@@ -62,9 +62,7 @@ int main (void)
 }
 
 /* { dg-final { scan-tree-dump-times "vect_recog_widen_shift_pattern: detected" 2 "vect" { target vect_widen_shift } } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 8} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 5} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/testsuite/gcc.dg/vect/vect-over-widen-4.c b/gcc/testsuite/gcc.dg/vect/vect-over-widen-4.c
index 3d536d5..0ef377f 100644
--- a/gcc/testsuite/gcc.dg/vect/vect-over-widen-4.c
+++ b/gcc/testsuite/gcc.dg/vect/vect-over-widen-4.c
@@ -66,9 +66,7 @@ int main (void)
 }
 
 /* { dg-final { scan-tree-dump-times "vect_recog_widen_shift_pattern: detected" 2 "vect" { target vect_widen_shift } } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 3} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 3} "vect" } } */
-/* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* << 8} "vect" } } */
 /* { dg-final { scan-tree-dump {vect_recog_over_widening_pattern: detected:[^\n]* >> 5} "vect" } } */
 /* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
 
diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index 8327e9d..3c54f04 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -5841,6 +5841,14 @@ vectorizable_shift (vec_info *vinfo,
     }
   vec_mode = TYPE_MODE (vectype);
   icode = (int) optab_handler (optab, vec_mode);
+
+  /* RTL expansion knows how to expand rotates using shift/or.  */
+  if (icode == CODE_FOR_nothing
+      && (code == LROTATE_EXPR || code == RROTATE_EXPR)
+      && optab_handler (ior_optab, vec_mode) != CODE_FOR_nothing
+      && optab_handler (ashl_optab, vec_mode) != CODE_FOR_nothing)
+    icode = (int) optab_handler (lshr_optab, vec_mode);
+
   if (icode == CODE_FOR_nothing)
     {
       if (dump_enabled_p ())