[C++ PATCH] Adjust -Wsequence-point for C++17 changes (PR c++/91415)

[C++ PATCH] Adjust -Wsequence-point for C++17 changes (PR c++/91415)

[Jakub Jelinek]

Hi!

The following patch adds some tweaks for -Wsequence-point warning for C++17
and later.  In particular, stop warning about no sequence point in between
<<, >>, ., -> and [] expressions, where E1 is in C++17 sequenced before E2.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

As mentioned in the PR, this is just part of the needed changes, I've tried
to adjust handling of MODIFY_EXPR, but didn't figure out exactly what needs
to be done, and .* / ->* aren't handled either, and CALL_EXPR needs probably
some verification too.

2019-08-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/91415
	* c-common.c (verify_tree): For LSHIFT_EXPR, RSHIFT_EXPR,
	COMPONENT_REF and ARRAY_REF in cxx_dialect >= cxx17 mode handle it
	like COMPOUND_EXPR rather than normal expression.

	* g++.dg/warn/sequence-pt-4.C: New test.

--- gcc/c-family/c-common.c.jj	2019-08-12 09:45:54.463491950 +0200
+++ gcc/c-family/c-common.c	2019-08-12 12:01:32.783135654 +0200
@@ -1889,6 +1889,7 @@ verify_tree (tree x, struct tlist **pbef
     case COMPOUND_EXPR:
     case TRUTH_ANDIF_EXPR:
     case TRUTH_ORIF_EXPR:
+    sequenced_binary:
       tmp_before = tmp_nosp = tmp_list2 = tmp_list3 = 0;
       verify_tree (TREE_OPERAND (x, 0), &tmp_before, &tmp_nosp, NULL_TREE);
       warn_for_collisions (tmp_nosp);
@@ -2031,8 +2035,18 @@ verify_tree (tree x, struct tlist **pbef
 	  x = TREE_OPERAND (x, 0);
 	  goto restart;
 	}
-      gcc_fallthrough ();
+      goto do_default;
+
+    case LSHIFT_EXPR:
+    case RSHIFT_EXPR:
+    case COMPONENT_REF:
+    case ARRAY_REF:
+      if (cxx_dialect >= cxx17)
+	goto sequenced_binary;
+      goto do_default;
+
     default:
+    do_default:
       /* For other expressions, simply recurse on their operands.
 	 Manual tail recursion for unary expressions.
 	 Other non-expressions need not be processed.  */
--- gcc/testsuite/g++.dg/warn/sequence-pt-4.C.jj	2019-08-12 12:16:27.205660149 +0200
+++ gcc/testsuite/g++.dg/warn/sequence-pt-4.C	2019-08-12 12:22:07.540530959 +0200
@@ -0,0 +1,21 @@
+/* More sequence point warning tests  */
+/* { dg-do compile } */
+/* { dg-options "-Wsequence-point" } */
+
+struct S { int a[10]; };
+void bar (int, int, int, int, int, int, int, int);
+
+int
+foo (int i, int x[10][10], int y[10], struct S z[10], struct S *w[10])
+{
+  int b = x[i++][i++];	/* { dg-warning "undefined" "sequence point warning" { target c++14_down } } */
+  int c = i++ << i++;	/* { dg-warning "undefined" "sequence point warning" { target c++14_down } } */
+  int d = i++ >> i++;	/* { dg-warning "undefined" "sequence point warning" { target c++14_down } } */
+  int e = i++ && i++;
+  int f = i++ ? i++ : i++;
+  int g = (i++, i++);
+  int h = z[i++].a[i++];	/* { dg-warning "undefined" "sequence point warning" { target c++14_down } } */
+  int j = w[i++]->a[i++];	/* { dg-warning "undefined" "sequence point warning" { target c++14_down } } */
+  bar (b, c, d, e, f,g, h, j);
+  return i;
+}

	Jakub

Re: [C++ PATCH] Adjust -Wsequence-point for C++17 changes (PR c++/91415)

[Jeff Law]

On 8/12/19 8:48 AM, Jakub Jelinek wrote:
> Hi!
> 
> The following patch adds some tweaks for -Wsequence-point warning for C++17
> and later.  In particular, stop warning about no sequence point in between
> <<, >>, ., -> and [] expressions, where E1 is in C++17 sequenced before E2.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> As mentioned in the PR, this is just part of the needed changes, I've tried
> to adjust handling of MODIFY_EXPR, but didn't figure out exactly what needs
> to be done, and .* / ->* aren't handled either, and CALL_EXPR needs probably
> some verification too.
> 
> 2019-08-12  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/91415
> 	* c-common.c (verify_tree): For LSHIFT_EXPR, RSHIFT_EXPR,
> 	COMPONENT_REF and ARRAY_REF in cxx_dialect >= cxx17 mode handle it
> 	like COMPOUND_EXPR rather than normal expression.
> 
> 	* g++.dg/warn/sequence-pt-4.C: New test.
OK
jeff