On 23/08/16 15:10, Wilco Dijkstra wrote:
> 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.
> To avoid this, limit the offset to +/-1GB so that the symbol needs to be
> within a
> 3GB 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.
>
> Bootstrap OK, updated tests now pass rather than failing with symbol out of
> range.
So isn't the real bug that we've permitted the user to create an object
that is too large for the data model?
Consider, for example:
char fixed_regs[0x200000000ULL];
char fixed_regs2[100];
int
main()
{
return fixed_regs[0] + fixed_regs2[0];
}
$ gcc -o test -mcmodel=small test2.c
/tmp/ccadJpSk.o: In function `main':
test2.c:(.text+0x10): relocation truncated to fit:
R_AARCH64_ADR_PREL_PG_HI21 against symbol `fixed_regs2' defined in
COMMON section in /tmp/ccadJpSk.o
collect2: error: ld returned 1 exit status
Neither offset is too large, but we still generate relocation errors
when trying to reference fixed_regs2.
R.
>
> OK for commit? As this is a latent bug, OK to backport to GCC6.x?
Not yet. And any back-port would need a PR first.
>
> ChangeLog:
> 2016-08-23 Wilco Dijkstra <wdijk...@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
> ab609223db7c54f467183db39c4f1ae8b789bfb5..60cc61a095bc144d602597a35b51b9a426c76c69
> 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -9367,24 +9367,22 @@ aarch64_classify_symbol (rtx x, rtx offset)
> 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. */
> + TINY code model. So we limit the maximum offset to +/-64KB and
> + assume the offset to the symbol is not larger than +/-(1M - 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))
> - || INTVAL (offset) < -1048575 || INTVAL (offset) > 1048575)
> + || !IN_RANGE (INTVAL (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. */
> + 1G, allowing +/-3G for the offset to the symbol. */
> if ((SYMBOL_REF_WEAK (x)
> && !aarch64_symbol_binds_local_p (x))
> - || !IN_RANGE (INTVAL (offset), HOST_WIDE_INT_C (-4294967263),
> - HOST_WIDE_INT_C (4294967264)))
> + || !IN_RANGE (INTVAL (offset), -0x40000000, 0x40000000))
> return SYMBOL_FORCE_TO_MEM;
> return SYMBOL_SMALL_ABSOLUTE;
>
> diff --git a/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c
> b/gcc/testsuite/gcc.target/aarch64/symbol-range-tiny.c
> index
> d7e46b059e41f2672b3a1da5506fa8944e752e01..d399a3637ed834ddc4bb429594c4ec229b5c2ea8
> 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..4b4ec8dab9321026d1fae96d336565a7883c8203
> 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\\\+" } } */
>
