[gcc r12-4471] Constant fold SS_NEG and SS_ABS in simplify-rtx.c

[gcc r12-4471] Constant fold SS_NEG and SS_ABS in simplify-rtx.c

[Roger Sayle]

https://gcc.gnu.org/g:94dff03f67fad1f9c526bb915bb21a4deb19d638

commit r12-4471-g94dff03f67fad1f9c526bb915bb21a4deb19d638
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Oct 18 11:51:07 2021 +0100

    Constant fold SS_NEG and SS_ABS in simplify-rtx.c
    
    This simple patch performs compile-time constant folding of
    signed saturating negation and signed saturating absolute value
    in the RTL optimizers.  Normally in two's complement arithmetic
    the lowest representable signed value overflows on negation,
    With these saturating operators they "saturate" to the maximum
    representable signed value, so SS_NEG:QI -128 is 127, and
    SS_ABS:HI -32768 is 32767.
    
    On bfin-elf, the following two short functions:
    
    short foo()
    {
      short t = -32768;
      short r = __builtin_bfin_negate_fr1x16(t);
      return r;
    }
    
    int bar()
    {
      int t = -2147483648;
      int r = __builtin_bfin_abs_fr1x32(t);
      return r;
    }
    
    currently compile to:
    _foo:   nop;
            nop;
            R0 = -32768 (X);
            R0 = -R0 (V);
            rts;
    
    _bar:   nop;
            R0 = -1 (X);
            R0 <<= 31;
            R0 = abs R0;
            rts;
    
    but with this middle-end patch now compile to:
    
    _foo:   nop;
            nop;
            nop;
            R0 = 32767 (X);
            rts;
    
    _bar:   nop;
            nop;
            R0 = -1 (X);
            R0.H = 32767;
            rts;
    
    2021-10-18  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            * simplify-rtx.c (simplify_const_unary_operation) [SS_NEG, SS_ABS]:
            Evalute SS_NEG and SS_ABS of a constant argument.
    
    gcc/testsuite/ChangeLog
            * gcc.target/bfin/ssabs.c: New test case.
            * gcc.target/bfin/ssneg.c: New test case.

Diff:
---
 gcc/simplify-rtx.c                    | 14 ++++++++++++++
 gcc/testsuite/gcc.target/bfin/ssabs.c | 11 +++++++++++
 gcc/testsuite/gcc.target/bfin/ssneg.c | 11 +++++++++++
 3 files changed, 36 insertions(+)

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index e4fae0b3e57..2bb18fb5065 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -2026,6 +2026,20 @@ simplify_const_unary_operation (enum rtx_code code, machine_mode mode,
 	  result = wide_int::from (op0, width, SIGNED);
 	  break;
 
+	case SS_NEG:
+	  if (wi::only_sign_bit_p (op0))
+	    result = wi::max_value (GET_MODE_PRECISION (imode), SIGNED);
+	  else
+	    result = wi::neg (op0);
+	  break;
+
+	case SS_ABS:
+	  if (wi::only_sign_bit_p (op0))
+	    result = wi::max_value (GET_MODE_PRECISION (imode), SIGNED);
+	  else
+	    result = wi::abs (op0);
+	  break;
+
 	case SQRT:
 	default:
 	  return 0;
diff --git a/gcc/testsuite/gcc.target/bfin/ssabs.c b/gcc/testsuite/gcc.target/bfin/ssabs.c
new file mode 100644
index 00000000000..e9d8bae62fd
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bfin/ssabs.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int foo()
+{
+  int t = -2147483648;
+  int r = __builtin_bfin_abs_fr1x32(t);
+  return r;
+}
+
+/* { dg-final { scan-assembler "32767" } } */
diff --git a/gcc/testsuite/gcc.target/bfin/ssneg.c b/gcc/testsuite/gcc.target/bfin/ssneg.c
new file mode 100644
index 00000000000..44ad7edbba4
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bfin/ssneg.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+short foo()
+{
+  short t = -32768;
+  short r = __builtin_bfin_negate_fr1x16(t);
+  return r;
+}
+
+/* { dg-final { scan-assembler "32767" } } */