RE: [X86 PATCH] Split lea into shorter left shift by 2 or 3 bits with -Oz.

["Roger Sayle" <roger@nextmovesoftware.com>]

Hi Uros,
Very many thanks for the speedy reviews.

Uros Bizjak wrote:
> On Thu, Oct 5, 2023 at 11:06 AM Roger Sayle <roger@nextmovesoftware.com>
> wrote:
> >
> >
> > This patch avoids long lea instructions for performing x<<2 and x<<3
> > by splitting them into shorter sal and move (or xchg instructions).
> > Because this increases the number of instructions, but reduces the
> > total size, its suitable for -Oz (but not -Os).
> >
> > The impact can be seen in the new test case:
> >
> > int foo(int x) { return x<<2; }
> > int bar(int x) { return x<<3; }
> > long long fool(long long x) { return x<<2; } long long barl(long long
> > x) { return x<<3; }
> >
> > where with -O2 we generate:
> >
> > foo:    lea    0x0(,%rdi,4),%eax        // 7 bytes
> >         retq
> > bar:    lea    0x0(,%rdi,8),%eax        // 7 bytes
> >         retq
> > fool:   lea    0x0(,%rdi,4),%rax        // 8 bytes
> >         retq
> > barl:   lea    0x0(,%rdi,8),%rax        // 8 bytes
> >         retq
> >
> > and with -Oz we now generate:
> >
> > foo:    xchg   %eax,%edi                // 1 byte
> >         shl    $0x2,%eax                // 3 bytes
> >         retq
> > bar:    xchg   %eax,%edi                // 1 byte
> >         shl    $0x3,%eax                // 3 bytes
> >         retq
> > fool:   xchg   %rax,%rdi                // 2 bytes
> >         shl    $0x2,%rax                // 4 bytes
> >         retq
> > barl:   xchg   %rax,%rdi                // 2 bytes
> >         shl    $0x3,%rax                // 4 bytes
> >         retq
> >
> > Over the entirety of the CSiBE code size benchmark this saves 1347
> > bytes (0.037%) for x86_64, and 1312 bytes (0.036%) with -m32.
> > Conveniently, there's already a backend function in i386.cc for
> > deciding whether to split an lea into its component instructions,
> > ix86_avoid_lea_for_addr, all that's required is an additional clause
> > checking for -Oz (i.e. optimize_size > 1).
> >
> > 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.  Additional testing was performed by repeating
> > these steps after removing the "optimize_size > 1" condition, so that
> > suitable lea instructions were always split [-Oz is not heavily
> > tested, so this invoked the new code during the bootstrap and
> > regression testing], again with no regressions.  Ok for mainline?
> >
> >
> > 2023-10-05  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386.cc (ix86_avoid_lea_for_addr): Split LEAs used
> >         to perform left shifts into shorter instructions with -Oz.
> >
> > gcc/testsuite/ChangeLog
> >         * gcc.target/i386/lea-2.c: New test case.
> >
> 
> OK, but ...
> 
> @@ -0,0 +1,7 @@
> +/* { dg-do compile { target { ! ia32 } } } */
> 
> Is there a reason to avoid 32-bit targets? I'd expect that the optimization also
> triggers on x86_32 for 32bit integers.

Good catch.  You're 100% correct; because the test case just checks that an LEA
is not used, and not for the specific sequence of shift instructions used instead,
this test also passes with --target_board='unix{-m32}'.  I'll remove the target clause
from the dg-do compile directive.

> +/* { dg-options "-Oz" } */
> +int foo(int x) { return x<<2; }
> +int bar(int x) { return x<<3; }
> +long long fool(long long x) { return x<<2; } long long barl(long long
> +x) { return x<<3; }
> +/* { dg-final { scan-assembler-not "lea\[lq\]" } } */

Thanks again.
Roger
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