Date: Mon, 22 Nov 2010 13:28:54 +0000 From: Julian Brown To: gcc-patches at gcc dot gnu dot org Cc: DJ Delorie Subject: [PATCH] Volatile bitfields vs. inline asm memory constraints Message-ID: <20101122132854.0aca431a@rex.config> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="MP_/ONpW806RnQ1ziaYj7_Y5E27" X-IsSubscribed: yes Mailing-List: contact gcc-patches-help at gcc dot gnu dot org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner at gcc dot gnu dot org Delivered-To: mailing list gcc-patches at gcc dot gnu dot org Hi, This patch fixes the issue in the (Launchpad, not GCC) bug tracker: https://bugs.launchpad.net/gcc-linaro/+bug/675347 The problem was introduced by the patch from DJ to honour volatile bitfield types: http://gcc.gnu.org/ml/gcc-patches/2010-06/msg01167.html but not exposed (on ARM) until the option was made the default (on the Linaro branch) -- it's not yet the default on mainline. The issue is as follows: after DJ's patch and with -fstrict-volatile-bitfields, in expr.c:expand_expr_real_1, the if condition with the comment "In cases where an aligned union has an unaligned object as a field, we might be extracting a BLKmode value from an integer-mode (e.g., SImode) object [...]" triggers for a normal (non-bitfield) volatile field of a struct/class. But, this appears to be over-eager: in the particular case mentioned above, when expanding a "volatile int" struct field used as a memory constraint for an inline asm, we end up with something which is no longer addressable (I think because of the actions of extract_bit_field). So, compilation aborts. My proposed fix is to restrict the conditional by only making it execute for -fstrict-volatile-bitfields only for non-naturally-aligned accesses: this appears to work (fixes test in question, and no regressions for cross to ARM Linux, gcc/g++/libstdc++, with -fstrict-volatile-bitfields turned on), but I don't know if there will be unintended consequences. DJ, does it look sane to you? Incidentally the constraints in the inline asm in the Launchpad testcase might be slightly dubious (attempting to force (mem (reg)) by using both "+m" (var) and "r" (&var) constraints), but replacing them with e.g.: asm volatile("0:\n" "ldrex %[newValue], %[_q_value]\n" "sub %[newValue], %[newValue], #1\n" "strex %[result], %[newValue], %[_q_value]\n" "teq %[result], #0\n" "bne 0b\n" : [newValue] "=&r" (newValue), [result] "=&r" (result) : [_q_value] "Q" (_q_value) : "cc", "memory"); still leads to a warning (not an error) with trunk and -fstrict-volatile-bitfields: atomic-changed.cc:24:35: warning: use of memory input without lvalue in asm operand 2 is deprecated [enabled by default] The warning goes away with the attached patch. So, I don't think the problem is purely that the original inline asm is invalid. OK to apply, or any comments? Julian ChangeLog gcc/ * expr.c (expand_expr_real_1): Only use BLKmode for volatile accesses which are not naturally aligned. Index: gcc-4_5-branch/gcc/expr.c =================================================================== --- gcc-4_5-branch.orig/gcc/expr.c 2010-12-23 00:42:11.690101002 -0800 +++ gcc-4_5-branch/gcc/expr.c 2010-12-24 15:07:39.400101000 -0800 @@ -9029,7 +9029,8 @@ && modifier != EXPAND_INITIALIZER) /* If the field is volatile, we always want an aligned access. */ - || (volatilep && flag_strict_volatile_bitfields > 0) + || (volatilep && flag_strict_volatile_bitfields > 0 + && (bitpos % GET_MODE_ALIGNMENT (mode) != 0)) /* If the field isn't aligned enough to fetch as a memref, fetch it as a bit field. */ || (mode1 != BLKmode