On Tue, Jun 7, 2011 at 15:59, Cedric Roux <cedric.roux@xxxxxxxxxx> wrote: > On 06/07/2011 03:47 PM, Cedric Roux wrote: >>> >>> Or compile with-fno-delete-null-pointer-checks >> >> forget about that. Sorry... > > might be that after all! > (really sorry for the noise) No problems: you pinpointed it . Adding -fno- will indeed generate code for both sides of the '||': if ( (g->a==0) || (g==NULL) ) 0: e59f3028 ldr r3, [pc, #40] ; 30 <test_function+0x30> 4: e5933000 ldr r3, [r3] 8: e5d32000 ldrb r2, [r3] c: e3520000 cmp r2, #0 10: 0a000003 beq 24 <test_function+0x24> 14: e3530000 cmp r3, #0 (this time r3 is tested against 0, as the C code says) And indeed this explains the issue: -O2 enables -fdelete-null-pointer-checks, which is described as follows: "Assume that programs cannot safely dereference null pointers, and that no code or data element resides there. This enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that if a pointer is checked after it has already been dereferenced, it cannot be null." So indeed, with -fdelete-null-pointer-checks *and* additional passes, the left-hand dereferences makes gcc consider g non-null, causing the right-hand test to become useless and deleting it. @Andrew: so this was not an undefined behavior case; rather, it was merely a case of not having read the gcc docs deep enough, for which I apologize. Thanks again! -- Albert ARIBAUD - Motorola Mobility France SAS
