[PATCH take #2] Some additional zero-extension related optimizations in simplify-rtx.

["Roger Sayle" <roger@nextmovesoftware.com>]

[-- Attachment #1: Type: text/plain, Size: 8375 bytes --]


Many thanks to Segher and Richard for pointing out that my removal
of optimizations of ABS(ABS(x)) and ABS(FFS(x)) in the original version
of this patch was incorrect, and my assumption that these would be 
subsumed by val_signbit_known_clear_p was mistaken.  That the
tests for ABS and FFS looked out of place, was not an indication that
they were not required, but that we were missing simplifications for
the related SS_ABS, PARITY, POPCOUNT, CLRSB, CLZ and CTZ etc.
To make up for this mistake, in this revised patch I've not only restored
the tests for ABS and FFS, but also added the many sibling RTX codes
that I'd also expect to see optimized here, such as ABS(PARITY(x)).

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures.  Ok for mainline?


2022-08-02  Roger Sayle  <roger@nextmovesoftware.com>
            Segher Boessenkool  <segher@kernel.crashing.org>
            Richard Sandiford  <richard.sandiford@arm.com>

gcc/ChangeLog
        * simplify_rtx.cc (simplify_unary_operation_1) <ABS>: Add
        optimizations for CLRSB, PARITY, POPCOUNT, SS_ABS, CLZ, CTZ
        and LSHIFTRT that are all positive to complement the existing
        FFS and (idempotent) ABS simplifications.
        <SIGN_EXTEND>: Canonicalize SIGN_EXTEND to ZERO_EXTEND when
        val_signbit_known_clear_p is true of the operand.
        Simplify sign extensions of SUBREG truncations of operands
        that are already suitably (zero) extended.
        <ZERO_EXTEND>: Simplify zero extensions of SUBREG truncations
        of operands that are already suitably zero extended.

Thanks in advance,
Roger
--

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: 02 August 2022 10:39
> To: Roger Sayle <roger@nextmovesoftware.com>
> Cc: gcc-patches@gcc.gnu.org; 'Segher Boessenkool'
> <segher@kernel.crashing.org>
> Subject: Re: [PATCH] Some additional zero-extension related optimizations
in
> simplify-rtx.
> 
> "Roger Sayle" <roger@nextmovesoftware.com> writes:
> > This patch implements some additional zero-extension and
> > sign-extension related optimizations in simplify-rtx.cc.  The original
> > motivation comes from PR rtl-optimization/71775, where in comment #2
> Andrew Pinski sees:
> >
> > Failed to match this instruction:
> > (set (reg:DI 88 [ _1 ])
> >     (sign_extend:DI (subreg:SI (ctz:DI (reg/v:DI 86 [ x ])) 0)))
> >
> > On many platforms the result of DImode CTZ is constrained to be a
> > small unsigned integer (between 0 and 64), hence the truncation to
> > 32-bits (using a SUBREG) and the following sign extension back to
> > 64-bits are effectively a no-op, so the above should ideally (often)
> > be simplified to "(set (reg:DI 88) (ctz:DI (reg/v:DI 86 [ x ]))".
> >
> > To implement this, and some closely related transformations, we build
> > upon the existing val_signbit_known_clear_p predicate.  In the first
> > chunk, nonzero_bits knows that FFS and ABS can't leave the sign-bit
> > bit set, so the simplification of of ABS (ABS (x)) and ABS (FFS (x))
> > can itself be simplified.
> 
> I think I misunderstood, but just in case: RTL ABS is well-defined for the
minimum
> integer (giving back the minimum integer), so we can't assume that ABS
leaves
> the sign bit clear.
> 
> Thanks,
> Richard
> 
> > The second transformation is that we can canonicalized SIGN_EXTEND to
> > ZERO_EXTEND (as in the PR 71775 case above) when the operand's
> > sign-bit is known to be clear.  The final two chunks are for
> > SIGN_EXTEND of a truncating SUBREG, and ZERO_EXTEND of a truncating
> > SUBREG respectively.  The nonzero_bits of a truncating SUBREG
> > pessimistically thinks that the upper bits may have an arbitrary value
> > (by taking the SUBREG), so we need look deeper at the SUBREG's operand
> > to confirm that the high bits are known to be zero.
> >
> > Unfortunately, for PR rtl-optimization/71775, ctz:DI on x86_64 with
> > default architecture options is undefined at zero, so we can't be sure
> > the upper bits of reg:DI 88 will be sign extended (all zeros or all
ones).
> > nonzero_bits knows this, so the above transformations don't trigger,
> > but the transformations themselves are perfectly valid for other
> > operations such as FFS, POPCOUNT and PARITY, and on other
> > targets/-march settings where CTZ is defined at zero.
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > and make -k check, both with and without --target_board=unix{-m32},
> > with no new failures.  Testing with CSiBE shows these transformations
> > trigger on several source files (and with -Os reduces the size of the
> > code).  Ok for mainline?
> >
> >
> > 2022-07-27  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * simplify_rtx.cc (simplify_unary_operation_1) <ABS>: Simplify
> >         test as both FFS and ABS result in nonzero_bits returning a
> >         mask that satisfies val_signbit_known_clear_p.
> >         <SIGN_EXTEND>: Canonicalize SIGN_EXTEND to ZERO_EXTEND when
> >         val_signbit_known_clear_p is true of the operand.
> >         Simplify sign extensions of SUBREG truncations of operands
> >         that are already suitably (zero) extended.
> >         <ZERO_EXTEND>: Simplify zero extensions of SUBREG truncations
> >         of operands that are already suitably zero extended.
> >
> >
> > Thanks in advance,
> > Roger
> > --
> >
> > diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc index
> > fa20665..e62bf56 100644
> > --- a/gcc/simplify-rtx.cc
> > +++ b/gcc/simplify-rtx.cc
> > @@ -1366,9 +1366,8 @@ simplify_context::simplify_unary_operation_1
> (rtx_code code, machine_mode mode,
> >  	break;
> >
> >        /* If operand is something known to be positive, ignore the ABS.
*/
> > -      if (GET_CODE (op) == FFS || GET_CODE (op) == ABS
> > -	  || val_signbit_known_clear_p (GET_MODE (op),
> > -					nonzero_bits (op, GET_MODE (op))))
> > +      if (val_signbit_known_clear_p (GET_MODE (op),
> > +				     nonzero_bits (op, GET_MODE (op))))
> >  	return op;
> >
> >        /* If operand is known to be only -1 or 0, convert ABS to NEG.
> > */ @@ -1615,6 +1614,24 @@ simplify_context::simplify_unary_operation_1
> (rtx_code code, machine_mode mode,
> >  	    }
> >  	}
> >
> > +      /* We can canonicalize SIGN_EXTEND (op) as ZERO_EXTEND (op) when
> > +         we know the sign bit of OP must be clear.  */
> > +      if (val_signbit_known_clear_p (GET_MODE (op),
> > +				     nonzero_bits (op, GET_MODE (op))))
> > +	return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op));
> > +
> > +      /* (sign_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
> > +      if (GET_CODE (op) == SUBREG
> > +	  && subreg_lowpart_p (op)
> > +	  && GET_MODE (SUBREG_REG (op)) == mode
> > +	  && is_a <scalar_int_mode> (mode, &int_mode)
> > +	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
> > +	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
> > +	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION
> (int_mode)
> > +	  && (nonzero_bits (SUBREG_REG (op), mode)
> > +	      & ~(GET_MODE_MASK (op_mode)>>1)) == 0)
> > +	return SUBREG_REG (op);
> > +
> >  #if defined(POINTERS_EXTEND_UNSIGNED)
> >        /* As we do not know which address space the pointer is referring
to,
> >  	 we can do this only if the target does not support different
> > pointer @@ -1765,6 +1782,18 @@
> simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode
> mode,
> >  				     op0_mode);
> >  	}
> >
> > +      /* (zero_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
> > +      if (GET_CODE (op) == SUBREG
> > +	  && subreg_lowpart_p (op)
> > +	  && GET_MODE (SUBREG_REG (op)) == mode
> > +	  && is_a <scalar_int_mode> (mode, &int_mode)
> > +	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
> > +	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
> > +	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION
> (int_mode)
> > +	  && (nonzero_bits (SUBREG_REG (op), mode)
> > +	      & ~GET_MODE_MASK (op_mode)) == 0)
> > +	return SUBREG_REG (op);
> > +
> >  #if defined(POINTERS_EXTEND_UNSIGNED)
> >        /* As we do not know which address space the pointer is referring
to,
> >  	 we can do this only if the target does not support different
> > pointer

[-- Attachment #2: patchxt3.txt --]
[-- Type: text/plain, Size: 4053 bytes --]

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index fa20665..b53272b 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -1366,11 +1366,57 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	break;
 
       /* If operand is something known to be positive, ignore the ABS.  */
-      if (GET_CODE (op) == FFS || GET_CODE (op) == ABS
-	  || val_signbit_known_clear_p (GET_MODE (op),
-					nonzero_bits (op, GET_MODE (op))))
+      if (val_signbit_known_clear_p (GET_MODE (op),
+				     nonzero_bits (op, GET_MODE (op))))
 	return op;
 
+      /* Using nonzero_bits doesn't (currently) work for modes wider than
+	 HOST_WIDE_INT, so the following transformations help simplify
+	 ABS for TImode and wider.  */
+      switch (GET_CODE (op))
+	{
+	case ABS:
+	case CLRSB:
+	case FFS:
+	case PARITY:
+	case POPCOUNT:
+	case SS_ABS:
+	  return op;
+
+	case CLZ:
+	  if (is_a <scalar_int_mode> (mode, &int_mode)
+	      && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT)
+	    {
+	      HOST_WIDE_INT val0;
+	      if (CLZ_DEFINED_VALUE_AT_ZERO (int_mode, val0)
+		  && (val0 >> (GET_MODE_PRECISION (int_mode) - 1)) == 0)
+		return op;
+	    }
+	  break;
+
+	case CTZ:
+	  if (is_a <scalar_int_mode> (mode, &int_mode)
+	      && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT)
+	    {
+	      HOST_WIDE_INT val0;
+	      if (CTZ_DEFINED_VALUE_AT_ZERO (int_mode, val0)
+		  && (val0 >> (GET_MODE_PRECISION (int_mode) - 1)) == 0)
+		return op;
+	    }
+	  break;
+
+	case LSHIFTRT:
+	  if (CONST_INT_P (XEXP (op, 1))
+	      && INTVAL (XEXP (op, 1)) > 0
+	      && is_a <scalar_int_mode> (mode, &int_mode)
+	      && INTVAL (XEXP (op, 1)) < GET_MODE_PRECISION (int_mode))
+	    return op;
+	  break;
+
+	default:
+	  break;
+	}
+
       /* If operand is known to be only -1 or 0, convert ABS to NEG.  */
       if (is_a <scalar_int_mode> (mode, &int_mode)
 	  && (num_sign_bit_copies (op, int_mode)
@@ -1615,6 +1661,24 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	    }
 	}
 
+      /* We can canonicalize SIGN_EXTEND (op) as ZERO_EXTEND (op) when
+         we know the sign bit of OP must be clear.  */
+      if (val_signbit_known_clear_p (GET_MODE (op),
+				     nonzero_bits (op, GET_MODE (op))))
+	return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op));
+
+      /* (sign_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
+      if (GET_CODE (op) == SUBREG
+	  && subreg_lowpart_p (op)
+	  && GET_MODE (SUBREG_REG (op)) == mode
+	  && is_a <scalar_int_mode> (mode, &int_mode)
+	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
+	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
+	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION (int_mode)
+	  && (nonzero_bits (SUBREG_REG (op), mode)
+	      & ~(GET_MODE_MASK (op_mode) >> 1)) == 0)
+	return SUBREG_REG (op);
+
 #if defined(POINTERS_EXTEND_UNSIGNED)
       /* As we do not know which address space the pointer is referring to,
 	 we can do this only if the target does not support different pointer
@@ -1765,6 +1829,18 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 				     op0_mode);
 	}
 
+      /* (zero_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
+      if (GET_CODE (op) == SUBREG
+	  && subreg_lowpart_p (op)
+	  && GET_MODE (SUBREG_REG (op)) == mode
+	  && is_a <scalar_int_mode> (mode, &int_mode)
+	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
+	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
+	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION (int_mode)
+	  && (nonzero_bits (SUBREG_REG (op), mode)
+	      & ~GET_MODE_MASK (op_mode)) == 0)
+	return SUBREG_REG (op);
+
 #if defined(POINTERS_EXTEND_UNSIGNED)
       /* As we do not know which address space the pointer is referring to,
 	 we can do this only if the target does not support different pointer