[Patch AArch64] Fixup floating point division with -march=armv8-a+nosimd

[Ramana Radhakrishnan]

Hi,

I received a private report from a customer that gcc was putting out 
calls to __divdf3 when compiling with +nosimd. When the reciprocal math 
support was added this was probably an oversight or a typo.

The canonical examples is :

    double
    foo (double x, double y)
      {
        return x / y;
      }

    with -march=armv8-a+nosimd

generates a function that calls __divdf3. Ofcourse on AArch64 we don't
have any software floating point and this causes issues.

There is also a problem in +nosimd that has existed since the dawn of 
time in the port with respect to long doubles (128 bit floating point), 
here the ABI and the compiler expect the presence of the SIMD unit as 
these parameters are passed in the vector registers. Thus while +nosimd 
tries to prevent the use of SIMD instructions in the compile we don't 
get this right as we end up using ldr qN / str qN instructions and even 
there I think things go wrong in a simple example that I tried.

Is that sufficient to consider marking +nosimd as deprecated in GCC-8 
and remove this in a future release ?

That is not a subject for this patch but something separate but I would 
like to put this into trunk and the release branches. Bootstrapped and 
regression tested on my aarch64 desktop.

Ok ?

regards
Ramana


    * config/aarch64/aarch64.md (div<mode>3): Change check to TARGET_FLOAT.
    * config/aarch64/aarch64.c (aarch64_emit_approx_div): Add early 
exit for vector mode and !TARGET_SIMD.
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 0c67e2b5c67..811f19c3f11 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -8442,6 +8442,9 @@ aarch64_emit_approx_div (rtx quo, rtx num, rtx den)
       || !use_approx_division_p)
     return false;
 
+  if (!TARGET_SIMD && VECTOR_MODE_P (mode))
+    return false;
+
   /* Estimate the approximate reciprocal.  */
   rtx xrcp = gen_reg_rtx (mode);
   emit_insn ((*get_recpe_type (mode)) (xrcp, den));
diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 423a3352aab..83e49425090 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -5093,7 +5093,7 @@
  [(set (match_operand:GPF_F16 0 "register_operand")
        (div:GPF_F16 (match_operand:GPF_F16 1 "general_operand")
                    (match_operand:GPF_F16 2 "register_operand")))]
- "TARGET_SIMD"
+ "TARGET_FLOAT"
 {
   if (aarch64_emit_approx_div (operands[0], operands[1], operands[2]))
     DONE;