Re: [PATCH][AArch64] Fix symbol offset limit

[Richard Sandiford <richard.sandiford@arm.com>]

Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
> ping
>
>     In aarch64_classify_symbol symbols are allowed full-range offsets on relocations.
>     This means the offset can use all of the +/-4GB offset, leaving no offset available
>     for the symbol itself.  This results in relocation overflow and link-time errors
>     for simple expressions like &global_char + 0xffffff00.

If global_char really is a char then isn't that UB?  I guess we still
have to compile it without error though...

>     To avoid this, limit the offset to +/-1MB so that the symbol needs to be within a
>     3.9GB offset from its references.  For the tiny code model use a 64KB offset, allowing
>     most of the 1MB range for code/data between the symbol and its references.

These new values seem a bit magical.  Would it work for the original
testcase to use FORCE_TO_MEM if !offset_within_block_p (x, offset)?

Richard

>     Bootstrapped on AArch64, passes regress, OK for commit?
>
>     ChangeLog:
>     2018-11-09  Wilco Dijkstra  <wdijkstr@arm.com>
>
>         gcc/
>             * config/aarch64/aarch64.c (aarch64_classify_symbol):
>             Apply reasonable limit to symbol offsets.
>
>         testsuite/
>             * gcc.target/aarch64/symbol-range.c (foo): Set new limit.
>             * gcc.target/aarch64/symbol-range-tiny.c (foo): Likewise.
>
>     --
>
>     diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
>     index 83453d03095018eddd1801e71ef3836849267444..0023cb37bbae5afe9387840c1bb6b43586d4fac2 100644
>     --- a/gcc/config/aarch64/aarch64.c
>     +++ b/gcc/config/aarch64/aarch64.c
>     @@ -13047,26 +13047,26 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT offset)
>                   the offset does not cause overflow of the final address.  But
>                   we have no way of knowing the address of symbol at compile time
>                   so we can't accurately say if the distance between the PC and
>     -            symbol + offset is outside the addressible range of +/-1M in the
>     -            TINY code model.  So we rely on images not being greater than
>     -            1M and cap the offset at 1M and anything beyond 1M will have to
>     -            be loaded using an alternative mechanism.  Furthermore if the
>     -            symbol is a weak reference to something that isn't known to
>     -            resolve to a symbol in this module, then force to memory.  */
>     +            symbol + offset is outside the addressible range of +/-1MB in the
>     +            TINY code model.  So we limit the maximum offset to +/-64KB and
>     +            assume the offset to the symbol is not larger than +/-(1MB - 64KB).
>     +            Furthermore force to memory if the symbol is a weak reference to
>     +            something that doesn't resolve to a symbol in this module.  */
>                if ((SYMBOL_REF_WEAK (x)
>                     && !aarch64_symbol_binds_local_p (x))
>     -             || !IN_RANGE (offset, -1048575, 1048575))
>     +             || !IN_RANGE (offset, -0x10000, 0x10000))
>                  return SYMBOL_FORCE_TO_MEM;
>     +
>                return SYMBOL_TINY_ABSOLUTE;
>
>              case AARCH64_CMODEL_SMALL:
>                /* Same reasoning as the tiny code model, but the offset cap here is
>     -            4G.  */
>     +            1MB, allowing +/-3.9GB for the offset to the symbol.  */
>                if ((SYMBOL_REF_WEAK (x)
>                     && !aarch64_symbol_binds_local_p (x))
>     -             || !IN_RANGE (offset, HOST_WIDE_INT_C (-4294967263),
>     -                           HOST_WIDE_INT_C (4294967264)))
>     +             || !IN_RANGE (offset, -0x100000, 0x100000))
>                  return SYMBOL_FORCE_TO_MEM;
>     +
>                return SYMBOL_SMALL_ABSOLUTE;
>
>              case AARCH64_CMODEL_TINY_PIC:
>     diff --git a/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c b/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c
>     index d7e46b059e41f2672b3a1da5506fa8944e752e01..d49ff4dbe5786ef6d343d2b90052c09676dd7fe5 100644
>     --- a/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c
>     +++ b/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c
>     @@ -1,12 +1,12 @@
>     -/* { dg-do compile } */
>     +/* { dg-do link } */
>      /* { dg-options "-O3 -save-temps -mcmodel=tiny" } */
>
>     -int fixed_regs[0x00200000];
>     +char fixed_regs[0x00200000];
>
>      int
>     -foo()
>     +main ()
>      {
>     -  return fixed_regs[0x00080000];
>     +  return fixed_regs[0x000ff000];
>      }
>
>      /* { dg-final { scan-assembler-not "adr\tx\[0-9\]+, fixed_regs\\\+" } } */
>     diff --git a/gcc/testsuite/gcc.target/aarch64/symbol-range.c b/gcc/testsuite/gcc.target/aarch64/symbol-range.c
>     index 6574cf4310430b847e77ea56bf8f20ef312d53e4..75c87c12f08004c153efc5192e5cfab566c089db 100644
>     --- a/gcc/testsuite/gcc.target/aarch64/symbol-range.c
>     +++ b/gcc/testsuite/gcc.target/aarch64/symbol-range.c
>     @@ -1,12 +1,12 @@
>     -/* { dg-do compile } */
>     +/* { dg-do link } */
>      /* { dg-options "-O3 -save-temps -mcmodel=small" } */
>
>     -int fixed_regs[0x200000000ULL];
>     +char fixed_regs[0x200000000ULL];
>
>      int
>     -foo()
>     +main ()
>      {
>     -  return fixed_regs[0x100000000ULL];
>     +  return fixed_regs[0xfffff000ULL];
>      }
>
>      /* { dg-final { scan-assembler-not "adrp\tx\[0-9\]+, fixed_regs\\\+" } } */