Jakub, I have a patch for PR51491.
Consider the following test-case, compiled with gcc -O2 -S f.c -fdump-tree-all: ... int f(void) { int tt = 0; int t = 4; { int a[t]; tt = g(a); tt += a[0]; } { int a[4]; tt += g(a); tt += a[0]; } return tt; } ... The vla a[t] is gimplified to an alloca_with_align, with a __builtin_stack_save/__builtin_stack_restore pair inserted at the scope boundaries. This alloca_with_align is folded by ccp into an array declaration, resulting in this representation at f.c.149t.optimized: ... f () { <unnamed-unsigned:8> D.1726[16]; int a[4]; int tt; int D.1722; int D.1721; void * saved_stack.2; int D.1719; <bb 2>: saved_stack.2_3 = __builtin_stack_save (); tt_18 = g (&D.1726); D.1719_19 = MEM[(int[0:D.1713] *)&D.1726][0]; tt_20 = D.1719_19 + tt_18; __builtin_stack_restore (saved_stack.2_3); D.1721_21 = g (&a); tt_22 = D.1721_21 + tt_20; D.1722_23 = a[0]; tt_24 = D.1722_23 + tt_22; a ={v} {CLOBBER}; return tt_24; } ... The patch inserts this clobber of D.1726 when alloca_with_align is folded: ... @@ -13,6 +13,7 @@ tt_18 = g (&D.1726); D.1719_19 = MEM[(int[0:D.1713] *)&D.1726][0]; tt_20 = D.1719_19 + tt_18; + D.1726 ={v} {CLOBBER}; __builtin_stack_restore (saved_stack.2_3); D.1721_21 = g (&a); tt_22 = D.1721_21 + tt_20; ... This allows D.1726 and a[4] to be mapped onto the same stack space, as can be seen in expand dump: ... Partition 0: size 16 align 16 D.1726 a ... Bootstrapped and reg-tested (ada inclusive) on x86_64. OK for stage3? Thanks, - Tom 2011-12-13 Tom de Vries <t...@codesourcery.com> PR tree-optimization/51491 * tree-ssa-ccp.c (insert_clobber_before_stack_restore): New function. (ccp_fold_stmt): Use insert_clobber_before_stack_restore after a successful fold_builtin_alloca_with_align. * gcc.dg/pr51491.c: New test.
Index: gcc/tree-ssa-ccp.c =================================================================== --- gcc/tree-ssa-ccp.c (revision 182098) +++ gcc/tree-ssa-ccp.c (working copy) @@ -1690,6 +1690,36 @@ evaluate_stmt (gimple stmt) return val; } +/* Try to find a BUILT_IN_STACK_RESTORE after gsi_stmt (I) and insert a clobber + of T before it. */ + +static void +insert_clobber_before_stack_restore (gimple_stmt_iterator i, tree t) +{ + bool found = false; + tree clobber; + + for (; !gsi_end_p (i); gsi_next (&i)) + { + gimple stmt = gsi_stmt (i); + + if (!gimple_call_builtin_p (stmt, BUILT_IN_STACK_RESTORE)) + continue; + + found = true; + break; + } + + /* We should try harder, but for now, just return. */ + if (!found) + return; + + clobber = build_constructor (TREE_TYPE (t), NULL); + TREE_THIS_VOLATILE (clobber) = 1; + gsi_insert_before (&i, gimple_build_assign (t, clobber), + GSI_SAME_STMT); +} + /* Detects a __builtin_alloca_with_align with constant size argument. Declares fixed-size array and returns the address, if found, otherwise returns NULL_TREE. */ @@ -1824,7 +1854,9 @@ ccp_fold_stmt (gimple_stmt_iterator *gsi if (new_rhs) { bool res = update_call_from_tree (gsi, new_rhs); + tree var = TREE_OPERAND (TREE_OPERAND (new_rhs, 0),0); gcc_assert (res); + insert_clobber_before_stack_restore (*gsi, var); return true; } } Index: gcc/testsuite/gcc.dg/pr51491.c =================================================================== --- /dev/null (new file) +++ gcc/testsuite/gcc.dg/pr51491.c (revision 0) @@ -0,0 +1,25 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fdump-rtl-expand" } */ + + +int g(int*); + +int f(void) +{ + int tt = 0; + int t = 4; + { + int a[t]; + tt = g(a); + tt += a[0]; + } + { + int a[4]; + tt += g(a); + tt += a[0]; + } + return tt; +} + +/* { dg-final { scan-rtl-dump-times "Partition" 1 "expand"} } */ +/* { dg-final { cleanup-rtl-dump "expand" } } */