| Message ID | 00b801d91a50$30a62140$91f263c0$@nextmovesoftware.com |
|---|---|
| State | New |
| Headers | show |
| Series | [x86] Use ix86_expand_clear in ix86_split_ashl. | expand |
On Wed, Dec 28, 2022 at 1:06 AM Roger Sayle <roger@nextmovesoftware.com> wrote: > > This patch is a one line change, to call ix86_expand_clear instead of > emit_move_insn with const0_rtx in ix86_split_ashl, allowing the backend > to use an xor instruction to clear a register if appropriate. > > The effect is demonstrated with the following function. > __int128 foo(__int128 x, unsigned long long b) { > return ((__int128)b << 72) + x; > } > > previously with -O2, GCC would generate > > foo: movl $0, %eax > salq $8, %rdx > addq %rdi, %rax > adcq %rsi, %rdx > ret > > with this patch, it now generates > > foo: xorl %eax, %eax > salq $8, %rdx > addq %rdi, %rax > adcq %rsi, %rdx > ret > > 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. It's an independent piece of a larger bug fix. > Ok for mainline? > > > 2022-12-28 Roger Sayle <roger@nextmovesoftware.com> > > gcc/ChangeLog > * config/i386/i386-expand.cc (ix86_split_ashl): Call > ix86_expand_clear to generate an xor instruction. > > gcc/testsuite/ChangeLog > * gcc.target/i386/ashlti3-1.c: New test case. OK. Thanks, Uros. > > > Thanks in advance, > Roger > -- >
diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc index b6cc76d..3eddbc9 100644 --- a/gcc/config/i386/i386-expand.cc +++ b/gcc/config/i386/i386-expand.cc @@ -6211,7 +6211,7 @@ ix86_split_ashl (rtx *operands, rtx scratch, machine_mode mode) if (count >= half_width) { emit_move_insn (high[0], low[1]); - emit_move_insn (low[0], const0_rtx); + ix86_expand_clear (low[0]); if (count > half_width) ix86_expand_ashl_const (high[0], count - half_width, mode); diff --git a/gcc/testsuite/gcc.target/i386/ashlti3-1.c b/gcc/testsuite/gcc.target/i386/ashlti3-1.c new file mode 100644 index 0000000..1cf1306 --- /dev/null +++ b/gcc/testsuite/gcc.target/i386/ashlti3-1.c @@ -0,0 +1,6 @@ +/* { dg-do compile { target int128 } } */ +/* { dg-options "-O2" } */ +__int128 foo(__int128 x, unsigned long long b) { + return ((__int128)b << 72) + x; +} +/* { dg-final { scan-assembler-not "\tmovl\[ \\t\]+\\\$0," } } */
This patch is a one line change, to call ix86_expand_clear instead of emit_move_insn with const0_rtx in ix86_split_ashl, allowing the backend to use an xor instruction to clear a register if appropriate. The effect is demonstrated with the following function. __int128 foo(__int128 x, unsigned long long b) { return ((__int128)b << 72) + x; } previously with -O2, GCC would generate foo: movl $0, %eax salq $8, %rdx addq %rdi, %rax adcq %rsi, %rdx ret with this patch, it now generates foo: xorl %eax, %eax salq $8, %rdx addq %rdi, %rax adcq %rsi, %rdx ret 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. It's an independent piece of a larger bug fix. Ok for mainline? 2022-12-28 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog * config/i386/i386-expand.cc (ix86_split_ashl): Call ix86_expand_clear to generate an xor instruction. gcc/testsuite/ChangeLog * gcc.target/i386/ashlti3-1.c: New test case. Thanks in advance, Roger --