Fix PR 93044: extra cast is not removed

In this case we are not removing convert to a bigger size
back to the same size (or smaller) if signedness does not
match.
For an example:
```
  signed char _1;
...
  _1 = *a_4(D);
  b_5 = (short unsigned int) _1;
  _2 = (unsigned char) b_5;
```
The inner cast is not needed and can be removed but was not.
The match pattern for removing the extra cast is overly
complex so decided to add a new case for rather than trying
to modify the current if statement here.

Committed as approved. Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	PR tree-optimization/93044
	* match.pd (nested int casts): A truncation (to the same size or smaller)
	can always remove the inner cast.

gcc/testsuite/ChangeLog:

	PR tree-optimization/93044
	* gcc.dg/tree-ssa/cast-1.c: New test.
	* gcc.dg/tree-ssa/cast-2.c: New test.

3 files changed, 34 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 74f0a84..cfd6ea0 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -4340,6 +4340,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	 && ! (final_ptr && inside_prec != inter_prec))
      (ocvt @0))
 
+   /* `(outer:M)(inter:N) a:O`
+      can be converted to `(outer:M) a`
+      if M <= O && N >= O. No matter what signedness of the casts,
+      as the final is either a truncation from the original or just
+      a sign change of the type. */
+   (if (inside_int && inter_int && final_int
+        && final_prec <= inside_prec
+	&& inter_prec >= inside_prec)
+    (convert @0))
+
     /* A truncation to an unsigned type (a zero-extension) should be
        canonicalized as bitwise and of a mask.  */
     (if (GIMPLE /* PR70366: doing this in GENERIC breaks -Wconversion.  */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/cast-1.c b/gcc/testsuite/gcc.dg/tree-ssa/cast-1.c
new file mode 100644
index 0000000..0f33ab5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/cast-1.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-optimized" } */
+
+
+void f(signed char *a, unsigned char *c)
+{
+  unsigned short b = *a;
+  *c = ((unsigned char)b);
+}
+
+
+/* { dg-final { scan-tree-dump-not "\\(short unsigned int\\)" "optimized"} } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/cast-2.c b/gcc/testsuite/gcc.dg/tree-ssa/cast-2.c
new file mode 100644
index 0000000..d665e92
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/cast-2.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-optimized" } */
+
+
+void f(signed short *a, unsigned char *c)
+{
+  unsigned long b = *a;
+  *c = ((unsigned char)b);
+}
+
+
+/* { dg-final { scan-tree-dump-not "\\(long unsigned int\\)" "optimized"} } */