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

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

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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\[lq\]" } } */