PR rtl-optimization/101617: Use neg/sbb in ix86_expand_int_movcc.

This patch resolves PR rtl-optimization/101617 where we should generate
the exact same code for (X ? -1 : 1) as we do for ((X ? -1 : 0) | 1).
The cause of the current difference on x86_64 is actually in
ix86_expand_int_movcc that doesn't know that negl;sbbl can be used
to create a -1/0 result depending on whether the input is zero/nonzero.

So for Andrew Pinski's test case:

int f1(int i)
{
  return i ? -1 : 1;
}

GCC currently generates:

f1:     cmpl    $1, %edi
        sbbl    %eax, %eax	// x ? 0 : -1
        andl    $2, %eax	// x ? 0 : 2
        subl    $1, %eax	// x ? -1 : 1
        ret

but with the attached patch, now generates:

f1:	negl    %edi
        sbbl    %eax, %eax	// x ? -1 : 0
        orl     $1, %eax	// x ? -1 : 1
        ret

To implement this I needed to add two expanders to i386.md to generate
the required instructions (in both SImode and DImode) matching the
pre-existing define_insns of the same name.

2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/101617
	* config/i386/i386-expand.cc (ix86_expand_int_movcc): Add a
	special case (indicated by negate_cc_compare_p) to generate a
	-1/0 mask using neg;sbb.
	* config/i386/i386.md (x86_neg<mode>_ccc): New define_expand
	to generate an *x86_neg<mode>_ccc instruction.
	(x86_mov<mode>cc_0_m1_neg): Likewise, a new define_expand to
	generate a *x86_mov<mode>cc_0_m1_neg instruction.

gcc/testsuite/ChangeLog
	PR rtl-optimization/101617
	* gcc.target/i386/pr101617.c: New test case.

diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc
index 8e9d2b6..617bee2 100644 (file)
--- a/gcc/config/i386/i386-expand.cc
+++ b/gcc/config/i386/i386-expand.cc
@@ -3109,6 +3109,7 @@ ix86_expand_int_movcc (rtx operands[])
   rtx compare_op;
   machine_mode mode = GET_MODE (operands[0]);
   bool sign_bit_compare_p = false;
+  bool negate_cc_compare_p = false;
   rtx op0 = XEXP (operands[1], 0);
   rtx op1 = XEXP (operands[1], 1);
   rtx op2 = operands[2];
@@ -3155,16 +3156,48 @@ ix86_expand_int_movcc (rtx operands[])
       HOST_WIDE_INT cf = INTVAL (op3);
       HOST_WIDE_INT diff;
 
+      if ((mode == SImode
+          || (TARGET_64BIT && mode == DImode))
+         && (GET_MODE (op0) == SImode
+             || (TARGET_64BIT && GET_MODE (op0) == DImode)))
+       {
+         /* Special case x != 0 ? -1 : y.  */
+         if (code == NE && op1 == const0_rtx && ct == -1)
+           {
+             negate_cc_compare_p = true;
+             std::swap (ct, cf);
+             code = EQ;
+           }
+         else if (code == EQ && op1 == const0_rtx && cf == -1)
+           negate_cc_compare_p = true;
+       }
+
       diff = ct - cf;
       /*  Sign bit compares are better done using shifts than we do by using
          sbb.  */
       if (sign_bit_compare_p
+         || negate_cc_compare_p
          || ix86_expand_carry_flag_compare (code, op0, op1, &compare_op))
        {
          /* Detect overlap between destination and compare sources.  */
          rtx tmp = out;
 
-          if (!sign_bit_compare_p)
+         if (negate_cc_compare_p)
+           {
+             if (GET_MODE (op0) == DImode)
+               emit_insn (gen_x86_negdi_ccc (gen_reg_rtx (DImode), op0));
+             else
+               emit_insn (gen_x86_negsi_ccc (gen_reg_rtx (SImode),
+                                             gen_lowpart (SImode, op0)));
+
+             tmp = gen_reg_rtx (mode);
+             if (mode == DImode)
+               emit_insn (gen_x86_movdicc_0_m1_neg (tmp));
+             else
+               emit_insn (gen_x86_movsicc_0_m1_neg (gen_lowpart (SImode,
+                                                                 tmp)));
+           }
+         else if (!sign_bit_compare_p)
            {
              rtx flags;
              bool fpcmp = false;

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 2598cf7..2b1d65b 100644 (file)
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -11241,6 +11241,14 @@
   [(set_attr "type" "negnot")
    (set_attr "mode" "<MODE>")])
 
+(define_expand "x86_neg<mode>_ccc"
+  [(parallel
+    [(set (reg:CCC FLAGS_REG)
+         (ne:CCC (match_operand:SWI48 1 "register_operand")
+                 (const_int 0)))
+     (set (match_operand:SWI48 0 "register_operand")
+         (neg:SWI48 (match_dup 1)))])])
+
 (define_insn "*negqi_ext<mode>_2"
   [(set (zero_extract:SWI248
          (match_operand:SWI248 0 "register_operand" "+Q")
@@ -20752,6 +20760,12 @@
    (set_attr "mode" "<MODE>")
    (set_attr "length_immediate" "0")])
 
+(define_expand "x86_mov<mode>cc_0_m1_neg"
+  [(parallel
+    [(set (match_operand:SWI48 0 "register_operand")
+         (neg:SWI48 (ltu:SWI48 (reg:CCC FLAGS_REG) (const_int 0))))
+     (clobber (reg:CC FLAGS_REG))])])
+
 (define_split
   [(set (match_operand:SWI48 0 "register_operand")
        (neg:SWI48

diff --git a/gcc/testsuite/gcc.target/i386/pr101617.c b/gcc/testsuite/gcc.target/i386/pr101617.c
new file mode 100644 (file)
index 0000000..503bf11
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr101617.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+int f(int i)
+{
+  int t = i ? -1 : 0;
+  return t | 1;
+}
+
+int f1(int i)
+{
+  int t = i ? -1 : 1;
+  return t;
+}
+
+/* { dg-final { scan-assembler-times "negl" 2 } } */
+/* { dg-final { scan-assembler-times "sbbl" 2 } } */
+/* { dg-final { scan-assembler-times "orl" 2 } } */
+/* { dg-final { scan-assembler-not "cmpl" } } */
+