Optimize double word negation of zero extended values on x86.

It's not uncommon for GCC to convert between a (zero or one) Boolean
value and a (zero or all ones) mask value, possibly of a wider type,
using negation.

Currently on x86_64, the following simple test case:
__int128 foo(unsigned long x) { return -(__int128)x; }

compiles with -O2 to:

        movq    %rdi, %rax
        xorl    %edx, %edx
        negq    %rax
        adcq    $0, %rdx
        negq    %rdx
        ret

with this patch, which adds an additional peephole2 to i386.md,
we instead generate the improved:

        movq    %rdi, %rax
        negq    %rax
        sbbq    %rdx, %rdx
        ret

[and likewise for the (DImode) long long version using -m32.]
A peephole2 is appropriate as the double word negation and the
operation providing the xor are typically only split after combine.

In fact, the new peephole2 sequence:
;; Convert:
;;   xorl %edx, %edx
;;   negl %eax
;;   adcl $0, %edx
;;   negl %edx
;; to:
;;   negl %eax
;;   sbbl %edx, %edx    // *x86_mov<mode>cc_0_m1

is nearly identical to (and placed immediately after) the existing:
;; Convert:
;;   mov %esi, %edx
;;   negl %eax
;;   adcl $0, %edx
;;   negl %edx
;; to:
;;   xorl %edx, %edx
;;   negl %eax
;;   sbbl %esi, %edx

One potential objection/concern is that "sbb? %reg,%reg" may possibly be
incorrectly perceived as a false register dependency on older hardware,
much like "xor? %reg,%reg" may be perceived as a false dependency on
really old hardware.  This doesn't currently appear to be a concern
for the i386 backend's *x86_move<mode>cc_0_m1 as shown by the following
test code:

int bar(unsigned int x, unsigned int y) {
  return x > y ? -1 : 0;
}

which currently generates a "naked" sbb:
        cmp     esi, edi
        sbb     eax, eax
        ret

If anyone does potentially encounter a stall, it would easy to add
a splitter or peephole2 controlled by a tuning flag to insert an additional
xor to break the false dependency chain (when not optimizing for size),
but I don't believe this is required on recent microarchitectures.

2022-05-24 Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (peephole2): Convert xor;neg;adc;neg,
	i.e. a double word negation of a zero extended operand, to
	neg;sbb.

gcc/testsuite/ChangeLog
	* gcc.target/i386/neg-zext-1.c: New test case for -m32.
	* gcc.target/i386/neg-zext-2.c: New test case for -m64.

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 050dee7..b9b8f78 100644 (file)
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -11040,6 +11040,46 @@
      (clobber (reg:CC FLAGS_REG))])]
   "ix86_expand_clear (operands[0]);")
 
+;; Convert:
+;;   xorl %edx, %edx
+;;   negl %eax
+;;   adcl $0, %edx
+;;   negl %edx
+;; to:
+;;   negl %eax
+;;   sbbl %edx, %edx   // *x86_mov<mode>cc_0_m1
+
+(define_peephole2
+  [(parallel
+    [(set (match_operand:SWI48 0 "general_reg_operand") (const_int 0))
+     (clobber (reg:CC FLAGS_REG))])
+   (parallel
+    [(set (reg:CCC FLAGS_REG)
+         (ne:CCC (match_operand:SWI48 1 "general_reg_operand") (const_int 0)))
+     (set (match_dup 1) (neg:SWI48 (match_dup 1)))])
+   (parallel
+    [(set (match_dup 0)
+         (plus:SWI48 (plus:SWI48
+                       (ltu:SWI48 (reg:CC FLAGS_REG) (const_int 0))
+                       (match_dup 0))
+                     (const_int 0)))
+     (clobber (reg:CC FLAGS_REG))])
+   (parallel
+    [(set (match_dup 0)
+         (neg:SWI48 (match_dup 0)))
+     (clobber (reg:CC FLAGS_REG))])]
+  "REGNO (operands[0]) != REGNO (operands[1])"
+  [(parallel
+    [(set (reg:CCC FLAGS_REG)
+         (ne:CCC (match_dup 1) (const_int 0)))
+     (set (match_dup 1) (neg:SWI48 (match_dup 1)))])
+   (parallel
+    [(set (match_dup 0)
+         (if_then_else:SWI48 (ltu:SWI48 (reg:CC FLAGS_REG) (const_int 0))
+                             (const_int -1)
+                             (const_int 0)))
+     (clobber (reg:CC FLAGS_REG))])])
+
 (define_insn "*neg<mode>_1"
   [(set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m")
        (neg:SWI (match_operand:SWI 1 "nonimmediate_operand" "0")))

diff --git a/gcc/testsuite/gcc.target/i386/neg-zext-1.c b/gcc/testsuite/gcc.target/i386/neg-zext-1.c
new file mode 100644 (file)
index 0000000..ec91fb1
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/neg-zext-1.c
@@ -0,0 +1,7 @@
+/* { dg-do compile { target ia32 } } */
+/* { dg-options "-O2" } */
+
+long long foo(unsigned int x) { return -(long long)x; }
+
+/* { dg-final { scan-assembler "sbb" } } */
+/* { dg-final { scan-assembler-not "adc" } } */

diff --git a/gcc/testsuite/gcc.target/i386/neg-zext-2.c b/gcc/testsuite/gcc.target/i386/neg-zext-2.c
new file mode 100644 (file)
index 0000000..a6ed077
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/neg-zext-2.c
@@ -0,0 +1,7 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O2" } */
+
+__int128 fool(unsigned long x) { return -(__int128)x; }
+
+/* { dg-final { scan-assembler "sbb" } } */
+/* { dg-final { scan-assembler-not "adc" } } */