On Friday 03 August 2007, Jonathan S. Shapiro wrote: > On Fri, 2007-08-03 at 12:46 -0400, Daniel Jacobowitz wrote: > > On Fri, Aug 03, 2007 at 05:24:28PM +0100, Dave Korn wrote: > > > I'm a bit surprised too. But it occurs to me that the ARM, unlike > > > the i386 and m68k, is bi-endian. Perhaps you can't actually know which > > > mode it's running in at compile time and /have/ to test it at runtime? > > > > No. You can't generate code in general that works on both! > > Then it seems very curious that the constant folding should fail on this > platform. Any idea what may be going on here?
You're exploiting a hole in the C aliasing rules by accessing a 32-bit int as type char. I tested several compilers (4.2, 4.1 and 3.4 x86, 4.2 m68k and 4.2 arm) and the only one that eliminated the comparison was 3.4-x86. My guess is that most optimization passes see this type punning, and leave well alone. You're probably relying on the low-level RTL optimizers to spot this, which depending on architecture may be too late for it to be provably safe. I'm fairly surprised it got caught at all. Paul
