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

Message ID 20210616074517.GW7746@tucnak
State New
Headers show
Series
  • stor-layout: Create DECL_BIT_FIELD_REPRESENTATIVE even for bitfields in unions [PR101062]
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Commit Message

Xionghu Luo via Gcc-patches June 16, 2021, 7:45 a.m.
Hi!

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.

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

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.


	Jakub

Comments

Richard Biener June 16, 2021, 9:54 a.m. | #1
On Wed, 16 Jun 2021, Jakub Jelinek wrote:

> Hi!

> 

> 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.

> 

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


OK.

Thanks,
Richard.

> 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.

> 

> --- gcc/stor-layout.c.jj	2021-03-30 18:11:52.537092233 +0200

> +++ gcc/stor-layout.c	2021-06-15 10:58:59.244353965 +0200

> @@ -2072,9 +2072,14 @@ finish_bitfield_representative (tree rep

>    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)

> --- gcc/testsuite/gcc.dg/pr101062.c.jj	2021-06-15 10:42:58.642919880 +0200

> +++ gcc/testsuite/gcc.dg/pr101062.c	2021-06-15 10:42:40.897171191 +0200

> @@ -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;

> +}

> 

> 	Jakub

> 

> 


-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)

Patch

--- gcc/stor-layout.c.jj	2021-03-30 18:11:52.537092233 +0200
+++ gcc/stor-layout.c	2021-06-15 10:58:59.244353965 +0200
@@ -2072,9 +2072,14 @@  finish_bitfield_representative (tree rep
   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)
--- gcc/testsuite/gcc.dg/pr101062.c.jj	2021-06-15 10:42:58.642919880 +0200
+++ gcc/testsuite/gcc.dg/pr101062.c	2021-06-15 10:42:40.897171191 +0200
@@ -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;
+}