Re: [PATCH] PR tree-optimization/105835: Two narrowing patterns for match.pd.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/5/2022 7:12 AM, Roger Sayle wrote:
> This patch resolves PR tree-optimization/105835, which is a code quality
> (dead code elimination) regression at -O1 triggered/exposed by a recent
> change to canonicalize X&-Y as X*Y.  The new (shorter) form exposes some
> missed optimization opportunities that can be handled by adding some
> extra simplifications to match.pd.
>
> One transformation is to simplify "(short)(x ? 65535 : 0)" into the
> equivalent "x ? -1 : 0", or more accurately x ? (short)-1 : (short)0",
> as INTEGER_CSTs record their type, and integer conversions can be
> pushed inside COND_EXPRs reducing the number of gimple statements.
>
> The other transformation is that (short)(X * 65535), where X is [0,1],
> into the equivalent (short)X * -1, (or again (short)-1 where tree's
> INTEGER_CSTs encode their type).  This is valid because multiplications
> where one operand is [0,1] are guaranteed not to overflow, and hence
> integer conversions can also be pushed inside these multiplications.
>
> These narrowing conversion optimizations can be identified by range
> analyses, such as EVRP, but these are only performed at -O2 and above,
> which is why this regression is only visible with -O1.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures.  Ok for mainline?
>
>
> 2022-06-05  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>          * match.pd (convert (mult zero_one_valued_p@1 INTEGER_CST@2)):
>          Narrow integer multiplication by a zero_one_valued_p operand.
>          (convert (cond @1 INTEGER_CST@2 INTEGER_CST@3)): Push integer
>          conversions inside COND_EXPR where both data operands are
>          integer constants.
>
> gcc/testsuite/ChangeLog
>          * gcc.dg/pr105835.c: New test case.
OK
jeff