[PATCH] ARM: do not emit lsrh/ashr for NEON shifts

[Tim Northover]

  Oh, nasty! Not quite as bad as it could be though. We range-check these intrinsics so at most one value is ever problematic. That suggests we can manage a less heavy-handed approach:


================
Comment at: lib/CodeGen/CGBuiltin.cpp:3089-3092
@@ -3077,7 +3088,6 @@
     return EmitNeonCall(CGM.getIntrinsic(Int, Ty), Ops, "vshll", 1);
   case ARM::BI__builtin_neon_vshl_n_v:
   case ARM::BI__builtin_neon_vshlq_n_v:
-    Ops[1] = EmitNeonShiftVector(Ops[1], Ty, false);
-    return Builder.CreateShl(Builder.CreateBitCast(Ops[0],Ty), Ops[1],
-                             "vshl_n");
+    Int = usgn ? Intrinsic::arm_neon_vshiftu : Intrinsic::arm_neon_vshifts;
+    return EmitNeonCall(CGM.getIntrinsic(Int, Ty), Ops, "vshl_n", 1, false);
   case ARM::BI__builtin_neon_vshrn_n_v:
----------------
All the documentation I can find says that vshl_n intrinsics take an integer from [0,B-1], which is in range for shift.

================
Comment at: lib/CodeGen/CGBuiltin.cpp:3096-3099
@@ -3085,10 +3095,6 @@
                         Ops, "vshrn_n", 1, true);
   case ARM::BI__builtin_neon_vshr_n_v:
   case ARM::BI__builtin_neon_vshrq_n_v:
-    Ops[0] = Builder.CreateBitCast(Ops[0], Ty);
-    Ops[1] = EmitNeonShiftVector(Ops[1], Ty, false);
-    if (usgn)
-      return Builder.CreateLShr(Ops[0], Ops[1], "vshr_n");
-    else
-      return Builder.CreateAShr(Ops[0], Ops[1], "vshr_n");
+    Int = usgn ? Intrinsic::arm_neon_vshiftu : Intrinsic::arm_neon_vshifts;
+    return EmitNeonCall(CGM.getIntrinsic(Int, Ty), Ops, "vshr_n", 1, true);
   case ARM::BI__builtin_neon_vsri_n_v:
----------------
The right shifts are more problematic. But the intrinsics again seem to be limited to [1, B] (and the value is known at compile-time).

Couldn't we detect the maximum value and emit 0 for an unsigned shift and "icmp lt %val, 0" for a signed shift?


http://llvm-reviews.chandlerc.com/D1819