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stor-layout: Create DECL_BIT_FIELD_REPRESENTATIVE even for bitfields in unions [PR101062]

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The following testcase is miscompiled on x86_64-linux, the bitfield store
is implemented as a RMW 64-bit operation at d+24 when the d variable has
size of only 28 bytes and scheduling moves in between the R and W part
a store to a different variable that happens to be right after the d
variable.

The reason for this is that we weren't creating
DECL_BIT_FIELD_REPRESENTATIVEs for bitfields in unions.

The following patch does create them, but treats all such bitfields as if
they were in a structure where the particular bitfield is the only field.

2021-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101062
	* stor-layout.c (finish_bitfield_representative): For fields in unions
	assume nextf is always NULL.
	(finish_bitfield_layout): Compute bit field representatives also in
	unions, but handle it as if each bitfield was the only field in the
	aggregate.

	* gcc.dg/pr101062.c: New test.
This commit is contained in:
Jakub Jelinek
2021-06-16 12:17:55 +02:00
parent 43fc4234ad
commit b4b50bf286
2 changed files with 44 additions and 11 deletions
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26
gcc/stor-layout.c
View File

@@ -2072,9 +2072,14 @@ finish_bitfield_representative (tree repr, tree field)
bitsize = (bitsize + BITS_PER_UNIT - 1) & ~(BITS_PER_UNIT - 1);
/* Now nothing tells us how to pad out bitsize ... */
nextf = DECL_CHAIN (field);
while (nextf && TREE_CODE (nextf) != FIELD_DECL)
nextf = DECL_CHAIN (nextf);
if (TREE_CODE (DECL_CONTEXT (field)) == RECORD_TYPE)
{
nextf = DECL_CHAIN (field);
while (nextf && TREE_CODE (nextf) != FIELD_DECL)
nextf = DECL_CHAIN (nextf);
}
else
nextf = NULL_TREE;
if (nextf)
{
tree maxsize;
@@ -2167,13 +2172,6 @@ finish_bitfield_layout (tree t)
tree field, prev;
tree repr = NULL_TREE;
/* Unions would be special, for the ease of type-punning optimizations
we could use the underlying type as hint for the representative
if the bitfield would fit and the representative would not exceed
the union in size. */
if (TREE_CODE (t) != RECORD_TYPE)
return;
for (prev = NULL_TREE, field = TYPE_FIELDS (t);
field; field = DECL_CHAIN (field))
{
@@ -2233,7 +2231,13 @@ finish_bitfield_layout (tree t)
if (repr)
DECL_BIT_FIELD_REPRESENTATIVE (field) = repr;
prev = field;
if (TREE_CODE (t) == RECORD_TYPE)
prev = field;
else if (repr)
{
finish_bitfield_representative (repr, field);
repr = NULL_TREE;
}
}
if (repr)

29
gcc/testsuite/gcc.dg/pr101062.c Normal file
View File

@@ -0,0 +1,29 @@
/* PR middle-end/101062 */
/* { dg-do run } */
/* { dg-options "-O2 -fno-toplevel-reorder -frename-registers" } */
union U { signed b : 5; };
int c;
volatile union U d[7] = { { 8 } };
short e = 1;
__attribute__((noipa)) void
foo ()
{
d[6].b = 0;
d[6].b = 0;
d[6].b = 0;
d[6].b = 0;
d[6].b = 0;
e = 0;
c = 0;
}
int
main ()
{
foo ();
if (e != 0)
__builtin_abort ();
return 0;
}