On June 27, 2020 10:58:56 AM GMT+02:00, Jakub Jelinek <ja...@redhat.com> wrote:
>Hi!
>
>For lp64 targets and int off ... ptr[off + 1]
>is lowered in pointer_sum to *(ptr + ((sizetype) off + (sizetype) 1)).
>That is fine when signed integer wrapping is undefined (and is not done
>already if off has unsigned type), but changes behavior for -fwrapv,
>where
>overflow is well defined. Runtime test could be:
>int
>main ()
>{
> char *p = __builtin_malloc (0x100000000UL);
> if (!p) return 0;
> char *q = p + 0x80000000UL;
> int o = __INT_MAX__;
> q[o + 1] = 1;
> if (q[-__INT_MAX__ - 1] != 1) __builtin_abort ();
> return 0;
>}
>with -fwrapv or so, not included in the testsuite because it requires
>4GB
>allocation (with some other test it would be enough to have something
>slightly above 2GB, but still...).
>
>Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Richard.
>On the other side, it would be nice with undefined signed integer
>overflow
>to optimize
>extern signed char arr[];
>int foo (int off) { off++; return arr[off]; }
>at least on targets that have addressing that can include both index
>and
>some immediate, right now we emit
> addl $1, %edi
> movslq %edi, %rdi
> movsbl arr(%rdi), %eax
>but we could emit
> movslq %edi, %rdi
> movsbl arr+1(%rdi), %eax
>Not sure where to do that; after expansion it is too late, because
>signed
>vs. unsigned addition is lost at that point, so maybe the isel pass or
>some
>other late pass? Wonder if ivopts is able to do that, but ivopts
>doesn't
>work outside of loops, right?
Right. SLSR was supposed to eventually handle this.
Richard.
>2020-06-27 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/95903
> * c-common.c (pointer_int_sum): Use TYPE_OVERFLOW_UNDEFINED instead of
> !TYPE_UNSIGNED check to see if we can apply distributive law and
>handle
> smaller precision intop operands separately.
>
> * c-c++-common/pr95903.c: New test.
>
>--- gcc/c-family/c-common.c.jj 2020-06-18 11:26:09.522400264 +0200
>+++ gcc/c-family/c-common.c 2020-06-26 10:28:10.385144734 +0200
>@@ -3141,7 +3141,7 @@ pointer_int_sum (location_t loc, enum tr
> /* If the constant is unsigned, and smaller than the pointer size,
> then we must skip this optimization. This is because it could cause
> an overflow error if the constant is negative but INTOP is not. */
>- && (!TYPE_UNSIGNED (TREE_TYPE (intop))
>+ && (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (intop))
> || (TYPE_PRECISION (TREE_TYPE (intop))
> == TYPE_PRECISION (TREE_TYPE (ptrop)))))
> {
>--- gcc/testsuite/c-c++-common/pr95903.c.jj 2020-06-26
>10:43:19.325937680 +0200
>+++ gcc/testsuite/c-c++-common/pr95903.c 2020-06-26 10:47:24.017382649
>+0200
>@@ -0,0 +1,19 @@
>+/* PR middle-end/95903 */
>+/* { dg-do compile { target lp64 } } */
>+/* { dg-options "-O2 -fwrapv -fdump-tree-optimized" } */
>+/* Verify that for -fwrapv the + 1 addition is performed in the
>parameter's
>+ type before sign extending it. */
>+/* { dg-final { scan-tree-dump-times "off_\[0-9]+\\\(D\\\) \\+ 1" 2
>"optimized" } } */
>+
>+char
>+foo (const char *ptr, int off)
>+{
>+ off += 1;
>+ return ptr[off];
>+}
>+
>+char
>+bar (const char *ptr, int off)
>+{
>+ return ptr[off + 1];
>+}
>
> Jakub
