nds32: support denormalized result through FP emulator

Currently, the nds32 FPU dose not support the arithmetic of denormalized
number. When the nds32 FPU finds the result of the instruction is a
denormlized number, the nds32 FPU considers it to be an underflow condition
and rounds the result to an appropriate number. It may causes some loss
of precision. This commit proposes a solution to re-execute the
instruction by the FPU emulator to enhance the precision. To transfer
calculations from user space to kernel space, this feature will enable
the underflow exception trap by default. Enabling this feature may cause
some side effects:
  1. Performance loss due to extra FPU exception
  2. Need another scheme to control real underflow trap
       A new parameter, UDF_trap, which is belong to FPU context is used
     to control underflow trap.

User can configure this feature via CONFIG_SUPPORT_DENORMAL_ARITHMETIC

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/nds32/Kconfig.cpu b/arch/nds32/Kconfig.cpu
index bb06a1b7eef0..6482ed877f97 100644
--- a/arch/nds32/Kconfig.cpu
+++ b/arch/nds32/Kconfig.cpu
@@ -28,6 +28,19 @@ config LAZY_FPU
 
 	  For nomal case, say Y.
 
+config SUPPORT_DENORMAL_ARITHMETIC
+	bool "Denormal arithmetic support"
+	depends on FPU
+	default n
+	help
+	  Say Y here to enable arithmetic of denormalized number. Enabling
+	  this feature can enhance the precision for tininess number.
+	  However, performance loss in float pointe calculations is
+	  possibly significant due to additional FPU exception.
+
+	  If the calculated tolerance for tininess number is not critical,
+	  say N to prevent performance loss.
+
 config HWZOL
 	bool "hardware zero overhead loop support"
 	depends on CPU_D10 || CPU_D15