[PATCH] Fix PR/63841: empty constructor doesn't zero-initialize

[Teresa Johnson]

Added testcase. Here is the new patch:

2014-11-12    <tejohnson@google.com>

gcc:
        PR tree-optimization/63841
        * tree.c (initializer_zerop): A constructor with no elements
        does not zero initialize.

gcc/testsuite:
        * g++.dg/tree-ssa/pr63841.C: New test.

Index: tree.c
===================================================================
--- tree.c      (revision 217190)
+++ tree.c      (working copy)
@@ -10330,6 +10330,8 @@ initializer_zerop (const_tree init)
       {
        unsigned HOST_WIDE_INT idx;

+        if (!CONSTRUCTOR_NELTS (init))
+          return false;
        FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (init), idx, elt)
          if (!initializer_zerop (elt))
            return false;
Index: testsuite/g++.dg/tree-ssa/pr63841.C
===================================================================
--- testsuite/g++.dg/tree-ssa/pr63841.C (revision 0)
+++ testsuite/g++.dg/tree-ssa/pr63841.C (working copy)
@@ -0,0 +1,38 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+#include <cstdio>
+#include <string>
+
+std::string __attribute__ ((noinline)) comp_test_write() {
+  std::string data;
+
+  for (int i = 0; i < 2; ++i) {
+    char b = 1 >> (i * 8);
+    data.append(&b, 1);
+  }
+
+  return data;
+}
+
+std::string __attribute__ ((noinline)) comp_test_write_good() {
+  std::string data;
+
+  char b;
+  for (int i = 0; i < 2; ++i) {
+    b = 1 >> (i * 8);
+    data.append(&b, 1);
+  }
+
+  return data;
+}
+
+int main() {
+  std::string good = comp_test_write_good();
+  printf("expected: %hx\n", *(short*)good.c_str());
+
+  std::string bad = comp_test_write();
+  printf("got: %hx\n", *(short*)bad.c_str());
+
+  return good != bad;
+}

On Wed, Nov 12, 2014 at 2:17 PM, Xinliang David Li <davidxl@google.com> wrote:
> missing test case?
>
> David
>
> On Wed, Nov 12, 2014 at 2:13 PM, Teresa Johnson <tejohnson@google.com> wrote:
>> This patch fixes an issue where tree-strlen was incorrectly removing a
>> store of 0 into a string because it thought a prior CLOBBER (which is
>> an empty constructor with no elements) was zero-initializing the
>> string.
>>
>> Bootstrapped and tested on x86_64-unknown-linux-gnu. OK for trunk?
>>
>> Thanks,
>> Teresa
>>
>> 2014-11-12    <tejohnson@google.com>
>>
>>         PR tree-optimization/63841
>>         * tree.c (initializer_zerop): A constructor with no elements
>>         does not zero initialize.
>>
>> Index: tree.c
>> ===================================================================
>> --- tree.c      (revision 217190)
>> +++ tree.c      (working copy)
>> @@ -10330,6 +10330,8 @@ initializer_zerop (const_tree init)
>>        {
>>         unsigned HOST_WIDE_INT idx;
>>
>> +        if (!CONSTRUCTOR_NELTS (init))
>> +          return false;
>>         FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (init), idx, elt)
>>           if (!initializer_zerop (elt))
>>             return false;
>>
>>
>> --
>> Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413



-- 
Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413