[gcc r13-4913] Use ix86_expand_clear in ix86_split_ashl.

[gcc r13-4913] Use ix86_expand_clear in ix86_split_ashl.

[Roger Sayle]

https://gcc.gnu.org/g:38b649ec16c965733aab9efa6bf61faae3485d92

commit r13-4913-g38b649ec16c965733aab9efa6bf61faae3485d92
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Dec 28 19:27:52 2022 +0000

    Use ix86_expand_clear in ix86_split_ashl.
    
    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.

Diff:
---
 gcc/config/i386/i386-expand.cc            | 2 +-
 gcc/testsuite/gcc.target/i386/ashlti3-1.c | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc
index b6cc76d8a9c..3eddbc94360 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 00000000000..1cf1306ea02
--- /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," } } */