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
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.
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);
--- /dev/null
+/* { 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," } } */