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

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

[Roger Sayle]

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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.


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diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 477e6ce..9557bff 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -15543,6 +15543,13 @@ ix86_avoid_lea_for_addr (rtx_insn *insn, rtx operands[])
       && (regno0 == regno1 || regno0 == regno2))
     return true;
 
+  /* Split with -Oz if the encoding requires fewer bytes.  */
+  if (optimize_size > 1
+      && parts.scale > 1
+      && !parts.base
+      && (!parts.disp || parts.disp == const0_rtx)) 
+    return true;
+
   /* Check we need to optimize.  */
   if (!TARGET_AVOID_LEA_FOR_ADDR || optimize_function_for_size_p (cfun))
     return false;
diff --git a/gcc/testsuite/gcc.target/i386/lea-2.c b/gcc/testsuite/gcc.target/i386/lea-2.c
new file mode 100644
index 0000000..20aded8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/lea-2.c
@@ -0,0 +1,7 @@
+/* { dg-do compile { target { ! ia32 } } } */
+/* { 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\[lq\]" } } */

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

[Uros Bizjak]

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.

+/* { 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\[lq\]" } } */

Uros.

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

[Roger Sayle]

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
--