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i386: Split lea into shorter left shift by 2 or 3 bits with -Oz.

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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).

2023-10-06  Roger Sayle  <roger@nextmovesoftware.com>
	    Uros Bizjak  <ubizjak@gmail.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.
This commit is contained in:
Roger Sayle 2023-10-06 20:07:53 +01:00
parent c1bc7513b1
commit fa8c99c4a4
2 changed files with 14 additions and 0 deletions
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7
gcc/config/i386/i386.cc
View file

@ -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;

7
gcc/testsuite/gcc.target/i386/lea-2.c Normal file
View file

@ -0,0 +1,7 @@
/* { dg-do compile } */
/* { 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\[lq\]" } } */