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
Tested on x86_64-pc-linux-gnu with make bootstrap and make -k check,
both with and without --target_board='unix{-m32\ -march=cascadelake}'
with no new failures. Ok for mainline?
2022-02-28 Roger Sayle <ro...@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.
Thanks in advance,
Roger
--
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 8ffa641..4f082ee 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -11012,6 +11012,19 @@
[(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 0000000..24a60a9
--- /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" } } */
