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From: Hans-Joachim Widmaier <hjwidmai@foxboro.com> To: gcc-gnats@gcc.gnu.org Subject: target/7462: Bad code for unalign access on PPC Date: Wed, 31 Jul 2002 22:56:00 -0000 [thread overview] Message-ID: <200208010544.g715i6o14828@linux1> (raw) >Number: 7462 >Category: target >Synopsis: Bad code for unalign access on PPC >Confidential: no >Severity: serious >Priority: medium >Responsible: unassigned >State: open >Class: pessimizes-code >Submitter-Id: net >Arrival-Date: Wed Jul 31 22:56:02 PDT 2002 >Closed-Date: >Last-Modified: >Originator: >Release: 3.1.1 >Organization: Foxboro Eckardt Development GmbH >Environment: System: Linux linux1 2.4.10-4GB #1 Tue Sep 25 12:33:54 GMT 2001 i686 unknown Architecture: i686 host: i686-pc-linux-gnu build: i686-pc-linux-gnu target: powerpc-unknown-rtems configured with: /pub/build/gcc-3.1.1/configure --host=i686-pc-linux-gnu --target=powerpc-rtems --disable-nls --disable-shared --enable-languages=c --with-newlib --with-cpu=405 --enable-threads=rtems >Description: Unaligned accesses to short/long/float variables in memory generate code which does multiple byte loads/stores with masking and shifting instead of 1 halfword/word load/store. On processors that do not support unaligned accesses this is the correct thing to do. For those that do, it is worse than just creating bad code: if the variable happens to live in a little-endian memory region it will not be swapped and therefore be invalid. >How-To-Repeat: The code was compiled with: powerpc-rtems-gcc -fallow-single-precision -mno-strict-align -fsingle-precision-constant -mcpu=405 -mtune=405 -O2 -c align.c # 1 "align.c" # 1 "<built-in>" # 1 "<command line>" # 1 "align.c" #pragma pack(1) struct atest { char b; float f; long l; } ATest; # 26 "align.c" void bad(void) { ATest.f *= 6.5; ATest.l *= 6; } void ok(void) { *&ATest.f *= 6.5; *&ATest.l *= 6; } >Fix: The split-up into byte accesses is not done when the memory is accessed via a pointer as in function ok() above. >Release-Note: >Audit-Trail: >Unformatted:
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