From fa8c99c4a426750adb5137f601b6f96b13e0da5c Mon Sep 17 00:00:00 2001 From: Roger Sayle Date: Fri, 6 Oct 2023 20:07:53 +0100 Subject: [PATCH] i386: Split lea into shorter left shift by 2 or 3 bits with -Oz. 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 Uros Bizjak 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. --- gcc/config/i386/i386.cc | 7 +++++++ gcc/testsuite/gcc.target/i386/lea-2.c | 7 +++++++ 2 files changed, 14 insertions(+) create mode 100644 gcc/testsuite/gcc.target/i386/lea-2.c diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc index 477e6cecc38..9557bffd092 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 00000000000..e9f12d7fb94 --- /dev/null +++ b/gcc/testsuite/gcc.target/i386/lea-2.c @@ -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\[lq\]" } } */