aarch64: Fix predication of FP8 FDOT insns [PR120986]

The predication of the SVE2 FP8 dot product insns was relying on the
architectural dependency:

FEAT_FP8DOT2 => FEAT_FP8DOT4

which was relaxed in GCC as of
r15-7480-g299a8e2dc667e795991bc439d2cad5ea5bd379e2, thus leading to
unrecognisable insn ICEs when compiling a two-way FDOT with just
+fp8dot2.  This patch introduces a new mode iterator which selectively
enables the appropriate mode(s) depending on which of the FP8DOT{2,4}
features are available, and uses it to fix the predication of the
patterns.

gcc/ChangeLog:

	PR target/120986
	* config/aarch64/aarch64-sve2.md (@aarch64_sve_dot<mode>):
	Switch mode iterator from SVE_FULL_HSF to new iterator;
	remove insn predicate as this is now taken care of by conditions
	in the mode iterator.
	(@aarch64_sve_dot_lane<mode>): Likewise.
	* config/aarch64/iterators.md (SVE_FULL_HSF_FP8_FDOT): New.

gcc/testsuite/ChangeLog:

	PR target/120986
	* gcc.target/aarch64/pr120986-1.c: New test.

3 files changed, 24 insertions, 8 deletions

diff --git a/gcc/config/aarch64/aarch64-sve2.md b/gcc/config/aarch64/aarch64-sve2.md
index a4c3257..a3cbbce 100644
--- a/gcc/config/aarch64/aarch64-sve2.md
+++ b/gcc/config/aarch64/aarch64-sve2.md
@@ -2211,14 +2211,14 @@
 ;; - FDOT (2-way, indexed) (FP8DOT2)
 ;; -------------------------------------------------------------------------
 (define_insn "@aarch64_sve_dot<mode>"
-  [(set (match_operand:SVE_FULL_HSF 0 "register_operand")
-	(unspec:SVE_FULL_HSF
-	  [(match_operand:SVE_FULL_HSF 1 "register_operand")
+  [(set (match_operand:SVE_FULL_HSF_FP8_FDOT 0 "register_operand")
+	(unspec:SVE_FULL_HSF_FP8_FDOT
+	  [(match_operand:SVE_FULL_HSF_FP8_FDOT 1 "register_operand")
 	   (match_operand:VNx16QI 2 "register_operand")
 	   (match_operand:VNx16QI 3 "register_operand")
 	   (reg:DI FPM_REGNUM)]
 	  UNSPEC_DOT_FP8))]
-  "TARGET_SSVE_FP8DOT4 && !(<MODE>mode == VNx8HFmode && !TARGET_SSVE_FP8DOT2)"
+  ""
   {@ [ cons: =0 , 1 , 2 , 3 ; attrs: movprfx ]
      [ w        , 0 , w , w ; *              ] fdot\t%0.<Vetype>, %2.b, %3.b
      [ ?&w      , w , w , w ; yes            ] movprfx\t%0, %1\;fdot\t%0.<Vetype>, %2.b, %3.b
@@ -2226,15 +2226,15 @@
 )
 
 (define_insn "@aarch64_sve_dot_lane<mode>"
-  [(set (match_operand:SVE_FULL_HSF 0 "register_operand")
-	(unspec:SVE_FULL_HSF
-	  [(match_operand:SVE_FULL_HSF 1 "register_operand")
+  [(set (match_operand:SVE_FULL_HSF_FP8_FDOT 0 "register_operand")
+	(unspec:SVE_FULL_HSF_FP8_FDOT
+	  [(match_operand:SVE_FULL_HSF_FP8_FDOT 1 "register_operand")
 	   (match_operand:VNx16QI 2 "register_operand")
 	   (match_operand:VNx16QI 3 "register_operand")
 	   (match_operand:SI 4 "const_int_operand")
 	   (reg:DI FPM_REGNUM)]
 	  UNSPEC_DOT_LANE_FP8))]
-  "TARGET_SSVE_FP8DOT4 && !(<MODE>mode == VNx8HFmode && !TARGET_SSVE_FP8DOT2)"
+  ""
   {@ [ cons: =0 , 1 , 2 , 3 ; attrs: movprfx ]
      [ w        , 0 , w , y ; *              ] fdot\t%0.<Vetype>, %2.b, %3.b[%4]
      [ ?&w      , w , w , y ; yes            ] movprfx\t%0, %1\;fdot\t%0.<Vetype>, %2.b, %3.b[%4]
diff --git a/gcc/config/aarch64/iterators.md b/gcc/config/aarch64/iterators.md
index 8f8237e..68b080d 100644
--- a/gcc/config/aarch64/iterators.md
+++ b/gcc/config/aarch64/iterators.md
@@ -543,6 +543,12 @@
 ;; elements.
 (define_mode_iterator SVE_FULL_HSF [VNx8HF VNx4SF])
 
+;; Like SVE_FULL_HSF, but selectively enables those modes that are valid
+;; for the variant of the SVE2 FP8 FDOT instruction associated with that
+;; mode.
+(define_mode_iterator SVE_FULL_HSF_FP8_FDOT [(VNx4SF "TARGET_SSVE_FP8DOT4")
+					     (VNx8HF "TARGET_SSVE_FP8DOT2")])
+
 ;; Partial SVE floating-point vector modes that have 16-bit or 32-bit
 ;; elements.
 (define_mode_iterator SVE_PARTIAL_HSF [VNx2HF VNx4HF VNx2SF])
diff --git a/gcc/testsuite/gcc.target/aarch64/pr120986-1.c b/gcc/testsuite/gcc.target/aarch64/pr120986-1.c
new file mode 100644
index 0000000..e3bc56c
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr120986-1.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=armv8.2-a+sve2+fp8dot2" } */
+#include <arm_sve.h>
+
+/* This triggered an ICE with an unrecognizable insn due to incorrect gating of
+   the insn in the backend.  */
+svfloat16_t foo(svfloat16_t a, svmfloat8_t b, svmfloat8_t c, unsigned long fpm)
+{
+  return svdot_lane_fpm (a, b, c, 0, fpm);
+}