A common idiom for testing if a specific set of bits is set in a value
is to use "(X & Y) == Y", which on x86 results in an AND followed by a
CMP.  A slightly improved implementation is to instead use (~X & Y)==0,
that uses a NOT and a TEST (or ANDN where available); still two "fast"
instructions, but typically shorter especially if Y is an immediate
constant.  Because the above transformation would require more gimple
statements in SSA, and may only be a win on targets with flags registers,
it isn't performed by the middle-end, instead leaving this choice to
the backend.

As an example, here's the change in code generation for pr91400-1.c
[which now requires a tweak to its dg-final clauses].

Before:
        movl    __cpu_model+12(%rip), %eax
        andl    $68, %eax       // 3 bytes
        cmpl    $68, %eax       // 3 bytes
        sete    %al
        ret

After:
        movl    __cpu_model+12(%rip), %eax
        notl    %eax            // 2 bytes
        testb   $68, %al        // 2 bytes
        sete    %al
        ret


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?


2022-05-26  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (*test<mode>_not): New define_insn_and_split
	to split a combined "and;cmp" sequence into "not;test".

gcc/testsuite/ChangeLog
	* gcc.target/i386/pr91400-1.c: Update for improved code generation.
	* gcc.target/i386/pr91400-2.c: Likewise.
	* gcc.target/i386/testnot-1.c: New test case.
	* gcc.target/i386/testnot-2.c: Likewise.


Thanks in advance,
Roger
--