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Improve stack layout heuristic.
- From: Easwaran Raman <eraman at google dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Sun, 17 Apr 2011 12:39:14 -0700
- Subject: Improve stack layout heuristic.
Hi,
This patch impoves the heuristic used in assigning stack location to
stack variables. Currently, if there are 3 variables A, B and C with
their sizes in increasing order and A and C have a conflict, it will
put A and B in a partition and C in a separate partition with a total
frame size of sizeof(B) + sizeof(C). This patch puts B and C in the
same partition and A in a separate partition, with a total size of
sizeof(A) + sizeof(C). The other improvement is when
-fno-strict-aliasing is used. In that case, it is ok to share stack
slot for a variable whose type transitively contains a union. The
other change in this patch removes the field offset in struct
stack_var. Right now, the field is always 0 due to the way it is set
in partition_stack_vars.
Bootstraps and no test regressions on x86_64-unknown-linux. OK for trunk?
-Easwaran
2011-04-17 Easwaran Raman <eraman@google.com>
* gcc/testsuite/gcc.dg/stack-layout-1.c: New test.
* gcc/testsuite/gcc.dg/stack-layout-2.c: New test.
* gcc/cfgexpand.c (struct stack_var): Remove OFFSET...
(add_stack_var): ...and its reference here...
(expand_stack_vars): ...and here.
(add_alias_set_conflicts): Add conflict to union variables
only when strict aliasing is used.
(stack_var_cmp): Sort by descending order of size.
(partition_stack_vars): Change heuristic.
(union_stack_vars): Fix to refelect changes in
partition_stack_vars.
(dump_stack_var_partition): Add newline after each partition.
Index: gcc/testsuite/gcc.dg/stack-layout-1.c
===================================================================
--- gcc/testsuite/gcc.dg/stack-layout-1.c (revision 0)
+++ gcc/testsuite/gcc.dg/stack-layout-1.c (revision 0)
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-strict-aliasing -fdump-rtl-expand" } */
+union U {
+ int a;
+ float b;
+};
+struct A {
+ union U u1;
+ char a[100];
+};
+void bar (struct A *);
+void foo ()
+ {
+ {
+ struct A a;
+ bar (&a);
+ }
+ {
+ struct A a;
+ bar (&a);
+ }
+ }
+
+/* { dg-final { scan-rtl-dump-times "Partition" 1 "expand" } } */
+/* { dg-final { cleanup-rtl-dump "expand" } } */
Index: gcc/testsuite/gcc.dg/stack-layout-2.c
===================================================================
--- gcc/testsuite/gcc.dg/stack-layout-2.c (revision 0)
+++ gcc/testsuite/gcc.dg/stack-layout-2.c (revision 0)
@@ -0,0 +1,23 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-rtl-expand" } */
+void bar( char *);
+int foo()
+{
+ int i=0;
+ {
+ char a[8000];
+ bar(a);
+ i += a[0];
+ }
+ {
+ char a[8192];
+ char b[32];
+ bar(a);
+ i += a[0];
+ bar(b);
+ i += a[0];
+ }
+ return i;
+}
+/* { dg-final { scan-rtl-dump "size 8192" "expand" } } */
+/* { dg-final { scan-rtl-dump "size 32" "expand" } } */
Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c (revision 171954)
+++ gcc/cfgexpand.c (working copy)
@@ -158,11 +158,6 @@
/* The Variable. */
tree decl;
- /* The offset of the variable. During partitioning, this is the
- offset relative to the partition. After partitioning, this
- is relative to the stack frame. */
- HOST_WIDE_INT offset;
-
/* Initially, the size of the variable. Later, the size of the partition,
if this variable becomes it's partition's representative. */
HOST_WIDE_INT size;
@@ -267,7 +262,6 @@
v = &stack_vars[stack_vars_num];
v->decl = decl;
- v->offset = 0;
v->size = tree_low_cst (DECL_SIZE_UNIT (SSAVAR (decl)), 1);
/* Ensure that all variables have size, so that &a != &b for any two
variables that are simultaneously live. */
@@ -372,8 +366,9 @@
to elements will conflict. In case of unions we have
to be careful as type based aliasing rules may say
access to the same memory does not conflict. So play
- safe and add a conflict in this case. */
- || contains_union)
+ safe and add a conflict in this case when -fstrict-aliasing
+ is used. */
+ || (contains_union && flag_strict_aliasing))
add_stack_var_conflict (i, j);
}
}
@@ -403,9 +398,9 @@
return (int)largeb - (int)largea;
/* Secondary compare on size, decreasing */
- if (sizea < sizeb)
- return -1;
if (sizea > sizeb)
+ return -1;
+ if (sizea < sizeb)
return 1;
/* Tertiary compare on true alignment, decreasing. */
@@ -564,28 +559,18 @@
/* A subroutine of partition_stack_vars. The UNION portion of a UNION/FIND
partitioning algorithm. Partitions A and B are known to be non-conflicting.
- Merge them into a single partition A.
+ Merge them into a single partition A. */
- At the same time, add OFFSET to all variables in partition B. At the end
- of the partitioning process we've have a nice block easy to lay out within
- the stack frame. */
-
static void
-union_stack_vars (size_t a, size_t b, HOST_WIDE_INT offset)
+union_stack_vars (size_t a, size_t b)
{
- size_t i, last;
struct stack_var *vb = &stack_vars[b];
bitmap_iterator bi;
unsigned u;
- /* Update each element of partition B with the given offset,
- and merge them into partition A. */
- for (last = i = b; i != EOC; last = i, i = stack_vars[i].next)
- {
- stack_vars[i].offset += offset;
- stack_vars[i].representative = a;
- }
- stack_vars[last].next = stack_vars[a].next;
+ /* Add B to A's partition. */
+ stack_vars[b].next = stack_vars[a].next;
+ stack_vars[b].representative = a;
stack_vars[a].next = b;
/* Update the required alignment of partition A to account for B. */
@@ -596,7 +581,7 @@
if (vb->conflicts)
{
EXECUTE_IF_SET_IN_BITMAP (vb->conflicts, 0, u, bi)
- add_stack_var_conflict (a, stack_vars[u].representative);
+ add_stack_var_conflict (a, u);
BITMAP_FREE (vb->conflicts);
}
}
@@ -605,16 +590,13 @@
partitions constrained by the interference graph. The overall
algorithm used is as follows:
- Sort the objects by size.
+ Sort the objects by size in descending order.
For each object A {
S = size(A)
O = 0
loop {
Look for the largest non-conflicting object B with size <= S.
UNION (A, B)
- offset(B) = O
- O += size(B)
- S -= size(B)
}
}
*/
@@ -636,24 +618,23 @@
for (si = 0; si < n; ++si)
{
size_t i = stack_vars_sorted[si];
- HOST_WIDE_INT isize = stack_vars[i].size;
unsigned int ialign = stack_vars[i].alignb;
- HOST_WIDE_INT offset = 0;
- for (sj = si; sj-- > 0; )
+ /* Ignore objects that aren't partition representatives. If we
+ see a var that is not a partition representative, it must
+ have been merged earlier. */
+ if (stack_vars[i].representative != i)
+ continue;
+
+ for (sj = si + 1; sj < n; ++sj)
{
size_t j = stack_vars_sorted[sj];
- HOST_WIDE_INT jsize = stack_vars[j].size;
unsigned int jalign = stack_vars[j].alignb;
/* Ignore objects that aren't partition representatives. */
if (stack_vars[j].representative != j)
continue;
- /* Ignore objects too large for the remaining space. */
- if (isize < jsize)
- continue;
-
/* Ignore conflicting objects. */
if (stack_var_conflict_p (i, j))
continue;
@@ -664,25 +645,8 @@
!= (jalign * BITS_PER_UNIT <= MAX_SUPPORTED_STACK_ALIGNMENT))
continue;
- /* Refine the remaining space check to include alignment. */
- if (offset & (jalign - 1))
- {
- HOST_WIDE_INT toff = offset;
- toff += jalign - 1;
- toff &= -(HOST_WIDE_INT)jalign;
- if (isize - (toff - offset) < jsize)
- continue;
-
- isize -= toff - offset;
- offset = toff;
- }
-
/* UNION the objects, placing J at OFFSET. */
- union_stack_vars (i, j, offset);
-
- isize -= jsize;
- if (isize == 0)
- break;
+ union_stack_vars (i, j);
}
}
@@ -712,9 +676,8 @@
{
fputc ('\t', dump_file);
print_generic_expr (dump_file, stack_vars[j].decl, dump_flags);
- fprintf (dump_file, ", offset " HOST_WIDE_INT_PRINT_DEC "\n",
- stack_vars[j].offset);
}
+ fputc ('\n', dump_file);
}
}
@@ -863,10 +826,9 @@
partition. */
for (j = i; j != EOC; j = stack_vars[j].next)
{
- gcc_assert (stack_vars[j].offset <= stack_vars[i].size);
expand_one_stack_var_at (stack_vars[j].decl,
base, base_align,
- stack_vars[j].offset + offset);
+ offset);
}
}