Re: [PATCH] Improved handling of MINUS_EXPR in bit CCP.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Aug 12, 2021 at 11:52 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch improves the bit bounds for MINUS_EXPR during tree-ssa's
> conditional constant propagation (CCP) pass (and as an added bonus
> adds support for POINTER_DIFF_EXPR).
>
> The pessimistic assumptions made by the current algorithm are
> demonstrated by considering 1 - (x&1).  Intuitively this should
> have possible values 0 and 1, and therefore an unknown mask of 1.
> Alas by treating subtraction as a negation followed by addition,
> the second operand first becomes 0 or -1, with an unknown mask
> of all ones, which results in the addition containing no known bits.
>
> Improved bounds are achieved by using the same approach used for
> PLUS_EXPR, determining the result with the minimum number of borrows,
> the result from the maximum number of borrows, and examining the bits
> they have in common.  One additional benefit of this approach
> is that it is applicable to POINTER_DIFF_EXPR, where previously the
> negation of a pointer didn't/doesn't make sense.
>
> A more convincing example, where a transformation missed by .032t.cpp
> isn't caught a few passes later by .038t.evrp, is the expression
> (7 - (x&5)) & 2, which (in the new test case) currently survives the
> tree-level optimizers but with this patch is now simplified to the
> constant value 2.
>
> This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
> and "make -k check" with no new failures.
>
> Ok for mainline?

OK.

Thanks,
Richard.

> 2021-08-12  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * tree-ssa-ccp.c (bit_value_binop) [MINUS_EXPR]: Use same
>         algorithm as PLUS_EXPR to improve subtraction bit bounds.
>         [POINTER_DIFF_EXPR]: Treat as synonymous with MINUS_EXPR.
>
> gcc/testsuite/ChangeLog
>         * gcc.dg/tree-ssa/ssa-ccp-40.c: New test case.
>
>
> Roger
> --
> Roger Sayle
> NextMove Software
> Cambridge, UK
>