Fold sign of LSHIFT_EXPR to eliminate no-op conversions.

This short patch teaches fold that it is "safe" to change the sign
of a left shift, to reduce the number of type conversions in gimple.
As an example:

unsigned int foo(unsigned int i) {
  return (int)i << 8;
}

is currently optimized to:

unsigned int foo (unsigned int i)
{
  int i.0_1;
  int _2;
  unsigned int _4;

  <bb 2> [local count: 1073741824]:
  i.0_1 = (int) i_3(D);
  _2 = i.0_1 << 8;
  _4 = (unsigned int) _2;
  return _4;
}

with this patch, this now becomes:

unsigned int foo (unsigned int i)
{
  unsigned int _2;

  <bb 2> [local count: 1073741824]:
  _2 = i_1(D) << 8;
  return _2;
}

which generates exactly the same assembly language.  Aside from the
reduced memory usage, the real benefit is that no-op conversions tend
to interfere with many folding optimizations.  For example,

unsigned int bar(unsigned char i) {
    return (i ^ (i<<16)) | (i<<8);
}

currently gets (tangled in conversions and) optimized to:

unsigned int bar (unsigned char i)
{
  unsigned int _1;
  unsigned int _2;
  int _3;
  int _4;
  unsigned int _6;
  unsigned int _8;

  <bb 2> [local count: 1073741824]:
  _1 = (unsigned int) i_5(D);
  _2 = _1 * 65537;
  _3 = (int) i_5(D);
  _4 = _3 << 8;
  _8 = (unsigned int) _4;
  _6 = _2 | _8;
  return _6;
}

but with this patch, bar now optimizes down to:

unsigned int bar(unsigned char i)
{
  unsigned int _1;
  unsigned int _4;

  <bb 2> [local count: 1073741824]:
  _1 = (unsigned int) i_3(D);
  _4 = _1 * 65793;
  return _4;

}

2021-08-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* match.pd (shift transformations): Change the sign of an
	LSHIFT_EXPR if it reduces the number of explicit conversions.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-convlshift-1.c: New test case.
	* gcc.dg/fold-convlshift-2.c: New test case.

3 files changed, 49 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 0fcfd0e..978a1b0 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3385,6 +3385,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
     (if (integer_zerop (@2) || integer_all_onesp (@2))
      (cmp @0 @2)))))
 
+/* Both signed and unsigned lshift produce the same result, so use
+   the form that minimizes the number of conversions.  */
+(simplify
+ (convert (lshift:s@0 (convert:s@1 @2) INTEGER_CST@3))
+ (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
+      && INTEGRAL_TYPE_P (TREE_TYPE (@2))
+      && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type))
+  (lshift (convert @2) @3)))
+
 /* Simplifications of conversions.  */
 
 /* Basic strip-useless-type-conversions / strip_nops.  */
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-1.c b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
new file mode 100644
index 0000000..b6f57f8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned int i)
+{
+  int t1 = i;
+  int t2 = t1 << 8;
+  return t2;
+}
+
+int bar(int i)
+{
+  unsigned int t1 = i;
+  unsigned int t2 = t1 << 8;
+  return t2;
+}
+
+/* { dg-final { scan-tree-dump-not "\\(int\\)" "optimized" } } */
+/* { dg-final { scan-tree-dump-not "\\(unsigned int\\)" "optimized" } } */
+
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-2.c b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
new file mode 100644
index 0000000..f21358c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned char c)
+{
+  int t1 = c;
+  int t2 = t1 << 8;
+  return t2;
+}
+
+int bar(unsigned char c)
+{
+  unsigned int t1 = c;
+  unsigned int t2 = t1 << 8;
+  return t2;
+}
+
+/* { dg-final { scan-tree-dump-times "\\(int\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\\(unsigned int\\)" 1 "optimized" } } */
+