[gcc r12-1668] PR target/11877: Use xor to write zero to memory with -Os

[gcc r12-1668] PR target/11877: Use xor to write zero to memory with -Os

[Roger Sayle]

https://gcc.gnu.org/g:9cedbaab8e048b90ceb9ceef0d851385fae67cde

commit r12-1668-g9cedbaab8e048b90ceb9ceef0d851385fae67cde
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Jun 21 08:54:50 2021 +0100

    PR target/11877: Use xor to write zero to memory with -Os
    
    The following patch attempts to resolve PR target/11877 (without
    triggering PR/23102).  On x86_64, writing an SImode or DImode zero
    to memory uses an instruction encoding that is larger than first
    clearing a register (using xor) then writing that to memory.  Hence,
    after reload, the peephole2 pass can determine if there's a suitable
    free register, and if so, use that to shrink the code size with -Os.
    
    To improve code size, and avoid inserting a large number of xor
    instructions (PR target/23102), this patch makes use of peephole2's
    efficient pattern matching to use a single temporary for a run of
    consecutive writes.  In theory, one could do better still with a
    new target-specific pass, gated on -Os, to shrink these instructions
    (like stv), but that's probably overkill for the little remaining
    space savings.
    
    Evaluating this patch on the CSiBE benchmark (v2.1.1) results in a
    0.26% code size improvement (3715273 bytes down to 3705477) on x86_64
    with -Os [saving 1 byte every 400].  549 of 894 tests improve, two
    tests grow larger.  Analysis of these 2 pathological cases reveals
    that although peephole2's match_scratch prefers to use a call-clobbered
    register (to avoid requiring a new stack frame), very rarely this
    interacts with GCC's shrink wrapping optimization, which may previously
    have avoided saving/restoring a call clobbered register, such as %eax,
    in the calling function.
    
    2021-06-21  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            PR target/11877
            * config/i386/i386.md: New define_peephole2s to shrink writing
            1, 2 or 4 consecutive zeros to memory when optimizing for size.
    
    gcc/testsuite/ChangeLog
            PR target/11877
            * gcc.target/i386/pr11877.c: New test case.

Diff:
---
 gcc/config/i386/i386.md                 | 36 +++++++++++++++++++++++++++++++++
 gcc/testsuite/gcc.target/i386/pr11877.c | 16 +++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 48532eb7ddf..2333261a894 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -19357,6 +19357,42 @@
   ix86_expand_clear (operands[1]);
 })
 
+;; When optimizing for size, zeroing memory should use a register.
+(define_peephole2
+  [(match_scratch:SWI48 0 "r")
+   (set (match_operand:SWI48 1 "memory_operand" "") (const_int 0))
+   (set (match_operand:SWI48 2 "memory_operand" "") (const_int 0))
+   (set (match_operand:SWI48 3 "memory_operand" "") (const_int 0))
+   (set (match_operand:SWI48 4 "memory_operand" "") (const_int 0))]
+  "optimize_insn_for_size_p () && peep2_regno_dead_p (0, FLAGS_REG)"
+  [(set (match_dup 1) (match_dup 0))
+   (set (match_dup 2) (match_dup 0))
+   (set (match_dup 3) (match_dup 0))
+   (set (match_dup 4) (match_dup 0))]
+{
+  ix86_expand_clear (operands[0]);
+})
+
+(define_peephole2
+  [(match_scratch:SWI48 0 "r")
+   (set (match_operand:SWI48 1 "memory_operand" "") (const_int 0))
+   (set (match_operand:SWI48 2 "memory_operand" "") (const_int 0))]
+  "optimize_insn_for_size_p () && peep2_regno_dead_p (0, FLAGS_REG)"
+  [(set (match_dup 1) (match_dup 0))
+   (set (match_dup 2) (match_dup 0))]
+{
+  ix86_expand_clear (operands[0]);
+})
+
+(define_peephole2
+  [(match_scratch:SWI48 0 "r")
+   (set (match_operand:SWI48 1 "memory_operand" "") (const_int 0))]
+  "optimize_insn_for_size_p () && peep2_regno_dead_p (0, FLAGS_REG)"
+  [(set (match_dup 1) (match_dup 0))]
+{
+  ix86_expand_clear (operands[0]);
+})
+
 ;; Reload dislikes loading constants directly into class_likely_spilled
 ;; hard registers.  Try to tidy things up here.
 (define_peephole2
diff --git a/gcc/testsuite/gcc.target/i386/pr11877.c b/gcc/testsuite/gcc.target/i386/pr11877.c
new file mode 100644
index 00000000000..5a488cccf77
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr11877.c
@@ -0,0 +1,16 @@
+/* PR target/11877 */
+/* { dg-do compile } */
+/* { dg-options "-Os" } */
+
+void foo (long long *p)
+{
+  *p = 0;
+}
+
+void bar (int *p)
+{
+  *p = 0;
+}
+
+/* { dg-final { scan-assembler-times "xorl\[ \t\]" 2 } } */
+/* { dg-final { scan-assembler-not "\\\$0," } } */