simplify-rtx: Implement constant folding of SS_TRUNCATE, US_TRUNCATE

This patch implements RTL constant-folding for the SS_TRUNCATE and US_TRUNCATE codes.
The semantics are a clamping operation on the argument with the min and max of the narrow mode,
followed by a truncation. The signedness of the clamp and the min/max extrema is derived from
the signedness of the saturating operation.

We have a number of instructions in aarch64 that use SS_TRUNCATE and US_TRUNCATE to represent
their operations and we have pretty thorough runtime tests in gcc.target/aarch64/advsimd-intrinsics/vqmovn*.c.
With this patch the instructions are folded away at optimisation levels and the correctness checks still
pass.

Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.

gcc/ChangeLog:

	* simplify-rtx.cc (simplify_const_unary_operation):
	Handle US_TRUNCATE, SS_TRUNCATE.

1 files changed, 14 insertions, 0 deletions

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index 276be67..21b7eb48 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -2131,6 +2131,20 @@ simplify_const_unary_operation (enum rtx_code code, machine_mode mode,
 	  result = wide_int::from (op0, width, UNSIGNED);
 	  break;
 
+	case US_TRUNCATE:
+	case SS_TRUNCATE:
+	  {
+	    signop sgn = code == US_TRUNCATE ? UNSIGNED : SIGNED;
+	    wide_int nmax
+	      = wide_int::from (wi::max_value (width, sgn),
+				GET_MODE_PRECISION (imode), sgn);
+	    wide_int nmin
+	      = wide_int::from (wi::min_value (width, sgn),
+				GET_MODE_PRECISION (imode), sgn);
+	    result = wi::min (wi::max (op0, nmin, sgn), nmax, sgn);
+	    result = wide_int::from (result, width, sgn);
+	    break;
+	  }
 	case SIGN_EXTEND:
 	  result = wide_int::from (op0, width, SIGNED);
 	  break;