libphobos: RISC-V: Fix soft-float build errors with IEEE exception flags

Fix assembly errors:

.../libphobos/src/std/math.d: Assembler messages:.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr s2'make[8]: *** [Makefile:1119: std/math.lo] Error 1

triggered with the RISC-V lp64 multilib in a GCC build configured with 
`--enable-multilib --enable-languages=all --target=riscv64-linux-gnu'. 
This is due to unconditional explicit use of F extension instructions 
within inline assembly, to access IEEE exception flags.  The use of 
these instructions is not allowed when building for a soft-float ABI.

Correct the problem by wrapping said inline assembly into a conditional 
such that if `D_SoftFloat' is true, then reads from IEEE exception flags 
return 0 and writes are ignored instead, complementing r270522 
("libphobos: Add D support for RISC-V Linux"), which is an updated 
version of <https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00325.html>, 
where the problematic code has originated from.

libphobos/ChangeLog:

2019-05-02  Maciej W. Rozycki  <macro@wdc.com>

	* std/math.d (IeeeFlags.getIeeeFlags): Handle RISC-V soft-float ABI.
	(IeeeFlags.resetIeeeFlags): Likewise.
	(FloatingPointControl.getControlState): Likewise.
	(FloatingPointControl.setControlState): Likewise.

From-SVN: r270815

1 files changed, 33 insertions, 13 deletions

diff --git a/libphobos/src/std/math.d b/libphobos/src/std/math.d
index 7e6a97f..ff1633a 100644
--- a/libphobos/src/std/math.d
+++ b/libphobos/src/std/math.d
@@ -4767,12 +4767,17 @@ private:
             }
             else version (RISCV_Any)
             {
-                uint result = void;
-                asm pure nothrow @nogc
+                version (D_SoftFloat)
+                    return 0;
+                else
                 {
-                    "frflags %0" : "=r" result;
+                    uint result = void;
+                    asm pure nothrow @nogc
+                    {
+                        "frflags %0" : "=r" result;
+                    }
+                    return result;
                 }
-                return result;
             }
             else
                 assert(0, "Not yet supported");
@@ -4850,10 +4855,15 @@ private:
             }
             else version (RISCV_Any)
             {
-                uint newValues = 0x0;
-                asm pure nothrow @nogc
+                version (D_SoftFloat)
+                    return;
+                else
                 {
-                    "fsflags %0" : : "r" newValues;
+                    uint newValues = 0x0;
+                    asm pure nothrow @nogc
+                    {
+                        "fsflags %0" : : "r" newValues;
+                    }
                 }
             }
             else
@@ -5450,12 +5460,17 @@ private:
             }
             else version (RISCV_Any)
             {
-                ControlState cont;
-                asm pure nothrow @nogc
+                version (D_SoftFloat)
+                    return 0;
+                else
                 {
-                    "frcsr %0" : "=r" cont;
+                    ControlState cont;
+                    asm pure nothrow @nogc
+                    {
+                        "frcsr %0" : "=r" cont;
+                    }
+                    return cont;
                 }
-                return cont;
             }
             else
                 assert(0, "Not yet supported");
@@ -5544,9 +5559,14 @@ private:
             }
             else version (RISCV_Any)
             {
-                asm pure nothrow @nogc
+                version (D_SoftFloat)
+                    return;
+                else
                 {
-                    "fscsr %0" : : "r" (newState);
+                    asm pure nothrow @nogc
+                    {
+                        "fscsr %0" : : "r" (newState);
+                    }
                 }
             }
             else