When making sizetypes no longer sign-extended (they are unsigned)
we run into extract_muldiv_1 miscompiling the Ada RTS during
secondary stack initialization while folding sizes for an allocation.
From
((sizetype) (_GLOBAL.SZ4_system.secondary_stack (<PLACEHOLDER_EXPR struct
system__secondary_stack__chunk_id>.last, <PLACEHOLDER_EXPR struct
system__secondary_stack__chunk_id>.first) /[cl] 8) + 15 &
0x0fffffffffffffff0) + 32
we eventually generate 2305843009213704224. Oops.
This is because extract_multiv_1 happily transforms
(((10240 - (sizetype) first) + 1) * 8) /[cl] 8
through
((sizetype) first * 0x0fffffffffffffff8 + 81928) /[cl] 8
to
((sizetype) first * 2305843009213693951 + 10241)
and then substitute 1 for first.
Well, the comment for that folding is totally odd - of _course_
unsigned sizetype things can overflow (we hid that issue merely
by pretending all unsigned sizetype constants (yes, only constants)
are signed. Huh.)
Off it goes.
Bootstrap and regtest pending on x86_64-unknown-linux-gnu.
Richard.
2011-08-31 Richard Guenther <rguent...@suse.de>
* fold-const.c (extract_muldiv_1): Remove bogus TYPE_IS_SIZETYPE
special-casing.
Index: trunk/gcc/fold-const.c
===================================================================
--- trunk.orig/gcc/fold-const.c 2011-08-31 10:53:58.000000000 +0200
+++ trunk/gcc/fold-const.c 2011-08-31 10:45:09.000000000 +0200
@@ -5894,11 +5894,9 @@ extract_muldiv_1 (tree t, tree c, enum t
multiple of the other, in which case we replace this with either an
operation or CODE or TCODE.
- If we have an unsigned type that is not a sizetype, we cannot do
- this since it will change the result if the original computation
- overflowed. */
- if ((TYPE_OVERFLOW_UNDEFINED (ctype)
- || (TREE_CODE (ctype) == INTEGER_TYPE && TYPE_IS_SIZETYPE (ctype)))
+ If we have an unsigned type, we cannot do this since it will change
+ the result if the original computation overflowed. */
+ if (TYPE_OVERFLOW_UNDEFINED (ctype)
&& ((code == MULT_EXPR && tcode == EXACT_DIV_EXPR)
|| (tcode == MULT_EXPR
&& code != TRUNC_MOD_EXPR && code != CEIL_MOD_EXPR
