[gcc r12-7417] PR tree-optimization/91384: peephole2 to eliminate testl after negl.

[gcc r12-7417] PR tree-optimization/91384: peephole2 to eliminate testl after negl.

[Roger Sayle]

https://gcc.gnu.org/g:28068d1115648adcc08ae57372170f3277915a0d

commit r12-7417-g28068d1115648adcc08ae57372170f3277915a0d
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Feb 28 22:30:27 2022 +0000

    PR tree-optimization/91384: peephole2 to eliminate testl after negl.
    
    This patch is my proposed solution to PR tree-optimization/91384 which is
    a missed-optimization/code quality regression on x86_64.  The problematic
    idiom is "if (r = -a)" which is equivalent to both "r = -a; if (r != 0)"
    and alternatively "r = -a; if (a != 0)".  In this particular case, on
    x86_64, we prefer to use the condition codes from the negation, rather
    than require an explicit testl instruction.
    
    Unfortunately, combine can't help, as it doesn't attempt to merge pairs
    of instructions that share the same operand(s), only pairs/triples of
    instructions where the result of each instruction feeds the next.  But
    I doubt there's sufficient benefit to attempt this kind of "combination"
    (that wouldn't already be caught by the tree-ssa passes).
    
    Fortunately, it's relatively easy to fix this up (addressing the
    regression) during peephole2 to eliminate the unnecessary testl in:
    
            movl    %edi, %ebx
            negl    %ebx
            testl   %edi, %edi
            je      .L2
    
    2022-02-28  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            PR tree-optimization/91384
            * config/i386/i386.md (peephole2): Eliminate final testl insn
            from the sequence *movsi_internal, *negsi_1, *cmpsi_ccno_1 by
            transforming using *negsi_2 for the negation.
    
    gcc/testsuite/ChangeLog
            PR tree-optimization/91384
            * gcc.target/i386/pr91384.c: New test case.

Diff:
---
 gcc/config/i386/i386.md                 | 13 +++++++++++++
 gcc/testsuite/gcc.target/i386/pr91384.c | 20 ++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index e7c54901d75..5e0a980e7ae 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -11011,6 +11011,19 @@
   [(set_attr "type" "negnot")
    (set_attr "mode" "<MODE>")])
 
+;; Optimize *negsi_1 followed by *cmpsi_ccno_1 (PR target/91384)
+(define_peephole2
+  [(set (match_operand:SWI 0 "general_reg_operand")
+	(match_operand:SWI 1 "general_reg_operand"))
+   (parallel [(set (match_dup 0) (neg:SWI (match_dup 0)))
+	      (clobber (reg:CC FLAGS_REG))])
+   (set (reg:CCZ FLAGS_REG) (compare:CCZ (match_dup 1) (const_int 0)))]
+  ""
+  [(set (match_dup 0) (match_dup 1))
+   (parallel [(set (reg:CCZ FLAGS_REG)
+		   (compare:CCZ (neg:SWI (match_dup 0)) (const_int 0)))
+	      (set (match_dup 0) (neg:SWI (match_dup 0)))])])
+
 ;; Special expand pattern to handle integer mode abs
 
 (define_expand "abs<mode>2"
diff --git a/gcc/testsuite/gcc.target/i386/pr91384.c b/gcc/testsuite/gcc.target/i386/pr91384.c
new file mode 100644
index 00000000000..24a60a932ab
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr91384.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+void foo (void);
+void bar (void);
+
+int
+test (int a)
+{
+  int r;
+
+  if (r = -a)
+    foo ();
+  else
+    bar ();
+
+  return r;
+}
+
+/* { dg-final { scan-assembler-not "testl" } } */