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 <ro...@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.
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\[lq\]" } } */
