On Mon, Jun 18, 2012 at 9:51 PM, Jiří Hruška <ji...@fud.cz> wrote:
> Hi all,
>
> I have tracked down a bug which results in invalid code being
> generated for indexed TARGET_MEM_REF expressions during dominator
> optimization.
>
> The conditions are: accessing objects adjacent in memory in a loop (in
> order to generate the TARGET_MEM_REF gimple) and optimizing this tree
> item during dom optimization (to trigger folding). There might be
> another set of conditions which get to the same state through a
> different
>
> The problem is that get_ref_base_and_extent() for TARGET_MEM_REF with
> variable index sets `maxsize' to -1 to signal that "via index or
> index2, the whole object can be reached" and returns. But before that,
> if the target object is a declaration with known size and `maxsize' is
> -1, it is updated, which can be taken by the caller (if `maxsize'
> equals to basic `size') as possibility to fold the expression into a
> constant.
>
> Assuming I understood the code and comments right, the solution is
> then to really take a quick exit in the abovementioned "indexed" case
> instead of just breaking the loop and letting the rest of function
> change the `maxsize' parameter.
>
> A quick search did not reveal any existing ticket for this problem.
> The bug was originally found in GCC 4.6.1 while compiling x86 code
> under MinGW, which is what the attached simplified testcase is based
> upon (compilation with -O1 is OK, anything higher fails).
> GCC 3.4.6 seems unaffected.
> Also the relevant code parts seem unchanged in current trunk.
> Patched build of 4.7.1 survived bootstrap on x86_64-rhel fine.
>
> The attached patch and all changes provided therein are released to
> public domain and can be freely used or modified by anyone.
>
> (This is my first time dealing with GCC bowels, please excuse my
> superficial understanding of everything I have written above.)
The issue is that your testcase is invalid.
__attribute__((section(".rodata$int0"))) const int fooS = 0;
__attribute__((section(".rodata$int1"))) const int foo1 = 1;
__attribute__((section(".rodata$int2"))) const int foo2 = 2;
__attribute__((section(".rodata$int3"))) const int foo3 = 3;
__attribute__((section(".rodata$int4"))) const int fooE = 0;
...
int x = ret(*(&fooS + i));
this access is only ever valid for i == 0 as otherwise you are creating
a pointer that points outside of the object fooS.
Richard.
> Thanks,
> Jiri Hruska
