diff --git a/gcc/testsuite/c-c++-common/pr103798-1.c b/gcc/testsuite/c-c++-common/pr103798-1.c
new file mode 100644
index 00000000000..cd3edf569fc
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-1.c
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int
+f (char a)
+{
+   return  __builtin_memchr ("a", a, 1) == 0;
+}
+
+__attribute__ ((weak))
+int
+g (char a)
+{
+  return a != 'a';
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-10.c b/gcc/testsuite/c-c++-common/pr103798-10.c
new file mode 100644
index 00000000000..4677e9539fa
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-10.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-Os -fdump-tree-optimized -save-temps" } */
+
+int
+f (char a)
+{
+  return  __builtin_memchr ("ac", a, 1) == 0;
+}
+
+/* { dg-final { scan-assembler "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-2.c b/gcc/testsuite/c-c++-common/pr103798-2.c
new file mode 100644
index 00000000000..e7e99c3679e
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-2.c
@@ -0,0 +1,30 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+#include <string.h>
+
+__attribute__ ((weak))
+int
+f (int a)
+{
+   return memchr ("aE", a, 2) != NULL;
+}
+
+__attribute__ ((weak))
+int
+g (char a)
+{
+  return a == 'a' || a == 'E';
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i + 256) != g (i + 256))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-3.c b/gcc/testsuite/c-c++-common/pr103798-3.c
new file mode 100644
index 00000000000..ddcedc7e238
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-3.c
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int
+f (char a)
+{
+   return  __builtin_memchr ("aEgZ", a, 3) == 0;
+}
+
+__attribute__ ((weak))
+int
+g (char a)
+{
+  return a != 'a' && a != 'E' && a != 'g';
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-4.c b/gcc/testsuite/c-c++-common/pr103798-4.c
new file mode 100644
index 00000000000..00e8302a833
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-4.c
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int
+f (char a)
+{
+   return  __builtin_memchr ("aEgi", a, 4) != 0;
+}
+
+__attribute__ ((weak))
+int
+g (char a)
+{
+  return a == 'a' || a == 'E' || a == 'g' || a == 'i';
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-5.c b/gcc/testsuite/c-c++-common/pr103798-5.c
new file mode 100644
index 00000000000..0d6487a13df
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-5.c
@@ -0,0 +1,26 @@
+/* { dg-do run { target int128 } } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int f(char a)
+{
+   return  __builtin_memchr ("aEgiH", a, 5) == 0;
+}
+
+__attribute__ ((weak))
+int g(char a)
+{
+  return a != 'a' && a != 'E' && a != 'g' && a != 'i' && a != 'H';
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-6.c b/gcc/testsuite/c-c++-common/pr103798-6.c
new file mode 100644
index 00000000000..5ccb5ee66e0
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-6.c
@@ -0,0 +1,27 @@
+/* { dg-do run { target int128 } } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int f(char a)
+{
+   return  __builtin_memchr ("aEgiHx", a, 6) != 0;
+}
+
+__attribute__ ((weak))
+int g(char a)
+{
+  return (a == 'a' || a == 'E' || a == 'g' || a == 'i' || a == 'H'
+	  || a == 'x');
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-7.c b/gcc/testsuite/c-c++-common/pr103798-7.c
new file mode 100644
index 00000000000..40fd38257d1
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-7.c
@@ -0,0 +1,27 @@
+/* { dg-do run { target int128 } } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int f(char a)
+{
+   return  __builtin_memchr ("aEgiHjZ", a, 7) == 0;
+}
+
+__attribute__ ((weak))
+int g(char a)
+{
+  return (a != 'a' && a != 'E' && a != 'g' && a != 'i' && a != 'H'
+	  && a != 'j' && a != 'Z');
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i) != g (i))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-8.c b/gcc/testsuite/c-c++-common/pr103798-8.c
new file mode 100644
index 00000000000..0841b18cea4
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-8.c
@@ -0,0 +1,27 @@
+/* { dg-do run { target int128 } } */
+/* { dg-options "-O2 -fdump-tree-optimized -save-temps" } */
+
+__attribute__ ((weak))
+int f(int a)
+{
+   return  __builtin_memchr ("aEgiHx19ABC", a, 8) != 0;
+}
+
+__attribute__ ((weak))
+int g(char a)
+{
+  return (a == 'a' || a == 'E' || a == 'g' || a == 'i' || a == 'H'
+	  || a == 'x' || a == '1' || a == '9');
+}
+
+int
+main ()
+{
+ for (int i = 0; i < 255; i++)
+   if (f (i + 256) != g (i + 256))
+     __builtin_abort ();
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/testsuite/c-c++-common/pr103798-9.c b/gcc/testsuite/c-c++-common/pr103798-9.c
new file mode 100644
index 00000000000..c5f0f94a4b5
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr103798-9.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-Os -fdump-tree-optimized -save-temps" } */
+
+int
+f (char a)
+{
+  return  __builtin_memchr ("a", a, 1) == 0;
+}
+
+/* { dg-final { scan-assembler-not "memchr" } } */
diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
index 69567ab3275..28549c85f76 100644
--- a/gcc/tree-ssa-forwprop.cc
+++ b/gcc/tree-ssa-forwprop.cc
@@ -42,6 +42,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-dfa.h"
 #include "tree-ssa-propagate.h"
 #include "tree-ssa-dom.h"
+#include "tree-ssa-strlen.h"
 #include "builtins.h"
 #include "tree-cfgcleanup.h"
 #include "cfganal.h"
@@ -1177,6 +1178,15 @@ constant_pointer_difference (tree p1, tree p2)
    memcpy (p, "abcd   ", 7);
    call if the latter can be stored by pieces during expansion.
 
+   Optimize
+   memchr ("abcd", a, 4) == 0;
+   or
+   memchr ("abcd", a, 4) != 0;
+   to
+   (a == 'a' || a == 'b' || a == 'c' || a == 'd') == 0
+   or
+   (a == 'a' || a == 'b' || a == 'c' || a == 'd') != 0
+
    Also canonicalize __atomic_fetch_op (p, x, y) op x
    to __atomic_op_fetch (p, x, y) or
    __atomic_op_fetch (p, x, y) iop x
@@ -1193,8 +1203,71 @@ simplify_builtin_call (gimple_stmt_iterator *gsi_p, tree callee2)
     return false;
   stmt1 = SSA_NAME_DEF_STMT (vuse);
 
+  tree res;
+
   switch (DECL_FUNCTION_CODE (callee2))
     {
+    case BUILT_IN_MEMCHR:
+      if (gimple_call_num_args (stmt2) == 3
+	  && (res = gimple_call_lhs (stmt2)) != nullptr
+	  && use_in_zero_equality (res) != nullptr
+	  && CHAR_BIT == 8
+	  && BITS_PER_UNIT == 8)
+	{
+	  tree ptr = gimple_call_arg (stmt2, 0);
+	  if (TREE_CODE (ptr) != ADDR_EXPR
+	      || TREE_CODE (TREE_OPERAND (ptr, 0)) != STRING_CST)
+	    break;
+	  unsigned HOST_WIDE_INT slen
+	    = TREE_STRING_LENGTH (TREE_OPERAND (ptr, 0));
+	  /* It must be a non-empty string constant.  */
+	  if (slen < 2)
+	    break;
+	  /* For -Os, only simplify strings with a single character.  */
+	  if (!optimize_bb_for_speed_p (gimple_bb (stmt2))
+	      && slen > 2)
+	    break;
+	  tree size = gimple_call_arg (stmt2, 2);
+	  /* Size must be a constant which is <= UNITS_PER_WORD and
+	     <= the string length.  */
+	  if (TREE_CODE (size) != INTEGER_CST || integer_zerop (size))
+	    break;
+
+	  if (!tree_fits_uhwi_p (size))
+	    break;
+
+	  unsigned HOST_WIDE_INT sz = tree_to_uhwi (size);
+	  if (sz > UNITS_PER_WORD || sz >= slen)
+	    break;
+
+	  tree ch = gimple_call_arg (stmt2, 1);
+	  location_t loc = gimple_location (stmt2);
+	  if (!useless_type_conversion_p (char_type_node,
+					  TREE_TYPE (ch)))
+	    ch = fold_convert_loc (loc, char_type_node, ch);
+	  const char *p = TREE_STRING_POINTER (TREE_OPERAND (ptr, 0));
+	  unsigned int isize = sz;
+	  tree *op = new tree[isize];
+	  for (unsigned int i = 0; i < isize; i++)
+	    {
+	      op[i] = build_int_cst (char_type_node, p[i]);
+	      op[i] = fold_build2_loc (loc, EQ_EXPR, boolean_type_node,
+				       op[i], ch);
+	    }
+	  for (unsigned int i = isize - 1; i >= 1; i--)
+	    op[i - 1] = fold_convert_loc (loc, boolean_type_node,
+					  fold_build2_loc (loc,
+							   BIT_IOR_EXPR,
+							   boolean_type_node,
+							   op[i - 1],
+							   op[i]));
+	  res = fold_convert_loc (loc, TREE_TYPE (res), op[0]);
+	  gimplify_and_update_call_from_tree (gsi_p, res);
+	  delete[] op;
+	  return true;
+	}
+      break;
+
     case BUILT_IN_MEMSET:
       if (gimple_call_num_args (stmt2) != 3
 	  || gimple_call_lhs (stmt2)
diff --git a/gcc/tree-ssa-strlen.cc b/gcc/tree-ssa-strlen.cc
index 7b3e3899ea2..5afbae1b72e 100644
--- a/gcc/tree-ssa-strlen.cc
+++ b/gcc/tree-ssa-strlen.cc
@@ -3913,8 +3913,8 @@ strlen_pass::handle_builtin_memset (bool *zero_write)
    nonnull if and only RES is used in such expressions exclusively and
    in none other.  */
 
-static gimple *
-use_in_zero_equality (tree res, bool exclusive = true)
+gimple *
+use_in_zero_equality (tree res, bool exclusive)
 {
   gimple *first_use = NULL;
 
diff --git a/gcc/tree-ssa-strlen.h b/gcc/tree-ssa-strlen.h
index 8d155450db8..fdb4d9d7783 100644
--- a/gcc/tree-ssa-strlen.h
+++ b/gcc/tree-ssa-strlen.h
@@ -35,6 +35,8 @@ struct c_strlen_data;
 extern void get_range_strlen_dynamic (tree, gimple *, c_strlen_data *,
 				      pointer_query &);
 
+extern gimple *use_in_zero_equality (tree, bool = true);
+
 /* APIs internal to strlen pass.  Defined in gimple-ssa-sprintf.cc.  */
 extern bool handle_printf_call (gimple_stmt_iterator *, pointer_query &);
 
