[FPU] prevent GCC from emitting fused multiply-add instructions (#905)

Author: stnolting

docs/datasheet/cpu.adoc

@@ -680,10 +680,14 @@ register file-related load/store or move instructions. The `Zfinx` extension'S f
 via dedicated <<_floating_point_csrs>>.
 This ISA extension is implemented as multi-cycle ALU co-process (`rtl/core/neorv32_cpu_cp_fpu.vhd`).
 
-.Fused Multiply-Add and Division Instructions
+.Fused / Multiply-Add Instructions
 [WARNING]
-Fused multiply-add instructions `f[n]m[add/sub].s` are not supported!
-Division `fdiv.s` and square root `fsqrt.s` instructions are not supported yet!
+Fused multiply-add instructions `f[n]m[add/sub].s` are not supported. A special GCC switch is used to prevent the
+compiler from emitting contracted/fused floating-point operations (see <<_default_compiler_flags>>).
+
+.Division and Squarer Root Instructions
+[WARNING]
+Division `fdiv.s` and square root `fsqrt.s` instructions are not supported yet.
 
 .Subnormal Number
 [WARNING]

docs/datasheet/software.adoc

@@ -286,6 +286,7 @@ The makefile's `CC_OPTS` is exported as **define** to be available within a C pr
 | `-g`                  | Include debugging information/symbols in ELF.
 | `-mstrict-align`      | Unaligned memory accesses cannot be resolved by the hardware and require emulation.
 | `-mbranch-cost=10`    | Branching costs a lot of cycles.
+| `-ffp-contract=off`   | Do not allow contraction of floatind-point operations (no fused operations as they are not supported).
 |=======================
 
 :sectnums:

sw/common/common.mk

@@ -111,7 +111,7 @@ IMAGE_GEN = $(NEORV32_EXG_PATH)/image_gen
 
 # Compiler & linker flags
 CC_OPTS  = -march=$(MARCH) -mabi=$(MABI) $(EFFORT) -Wall -ffunction-sections -fdata-sections -nostartfiles -mno-fdiv
-CC_OPTS += -mstrict-align -mbranch-cost=10 -g -Wl,--gc-sections
+CC_OPTS += -mstrict-align -mbranch-cost=10 -g -Wl,--gc-sections -ffp-contract=off
 CC_OPTS += $(USER_FLAGS)
 LD_LIBS =  -lm -lc -lgcc
 LD_LIBS += $(USER_LIBS)
@@ -324,72 +324,45 @@ clean_all: clean
 # Show configuration
 # -----------------------------------------------------------------------------
 info:
-	@echo "------------------------------------------------------"
-	@echo "-- Project"
-	@echo "------------------------------------------------------"
-	@echo "Project folder:        $(shell basename $(CURDIR))"
-	@echo "Source files:          $(APP_SRC)"
-	@echo "Include folder(s):     $(APP_INC)"
+	@echo "******************************************************"
+	@echo "Project / Makfile Configuration"
+	@echo "******************************************************"
+	@echo "Project folder: $(shell basename $(CURDIR))"
+	@echo "Source files: $(APP_SRC)"
+	@echo "Include folder(s): $(APP_INC)"
 	@echo "ASM include folder(s): $(ASM_INC)"
-	@echo "------------------------------------------------------"
-	@echo "-- NEORV32"
-	@echo "------------------------------------------------------"
 	@echo "NEORV32 home folder (NEORV32_HOME): $(NEORV32_HOME)"
 	@echo "IMAGE_GEN: $(IMAGE_GEN)"
 	@echo "Core source files:"
 	@echo "$(CORE_SRC)"
 	@echo "Core include folder:"
 	@echo "$(NEORV32_INC_PATH)"
-	@echo "------------------------------------------------------"
-	@echo "-- Objects"
-	@echo "------------------------------------------------------"
 	@echo "Project object files:"
 	@echo "$(OBJ)"
-	@echo "------------------------------------------------------"
-	@echo "-- RISC-V CPU"
-	@echo "------------------------------------------------------"
-	@echo "MARCH:          $(MARCH)"
-	@echo "MABI:           $(MABI)"
-	@echo "------------------------------------------------------"
-	@echo "-- Toolchain"
-	@echo "------------------------------------------------------"
-	@echo "CC:             $(CC)"
-	@echo "OBJDUMP:        $(OBJDUMP)"
-	@echo "OBJCOPY:        $(OBJCOPY)"
-	@echo "SIZE:           $(SIZE)"
-	@echo "DEBUGGER:       $(GDB)"
-	@echo "------------------------------------------------------"
-	@echo "-- GDB Arguments"
-	@echo "------------------------------------------------------"
-	@echo "GDB_ARGS:       $(GDB_ARGS)"
-	@echo "------------------------------------------------------"
-	@echo "-- GHDL Run Arguments"
-	@echo "------------------------------------------------------"
-	@echo "GHDL_RUN_FLAGS: $(GHDL_RUN_FLAGS)"
-	@echo "------------------------------------------------------"
-	@echo "-- Libraries"
-	@echo "------------------------------------------------------"
 	@echo "LIBGCC:"
 	@$(CC) -print-libgcc-file-name
 	@echo "SEARCH-DIRS:"
 	@$(CC) -print-search-dirs
-	@echo "------------------------------------------------------"
-	@echo "-- Compiler Flags"
-	@echo "------------------------------------------------------"
-	@echo "USER_FLAGS:     $(USER_FLAGS)"
-	@echo "CC_OPTS:        $(CC_OPTS)"
-	@echo "------------------------------------------------------"
-	@echo "-- Libraries"
-	@echo "------------------------------------------------------"
-	@echo "USER_LIBS:      $(USER_LIBS)"
-	@echo "LD_LIBS:        $(LD_LIBS)"
+	@echo "USER_LIBS: $(USER_LIBS)"
+	@echo "LD_LIBS: $(LD_LIBS)"
+	@echo "MARCH: $(MARCH)"
+	@echo "MABI: $(MABI)"
+	@echo "CC: $(CC)"
+	@echo "OBJDUMP: $(OBJDUMP)"
+	@echo "OBJCOPY: $(OBJCOPY)"
+	@echo "SIZE: $(SIZE)"
+	@echo "DEBUGGER: $(GDB)"
+	@echo "GDB_ARGS: $(GDB_ARGS)"
+	@echo "GHDL_RUN_FLAGS: $(GHDL_RUN_FLAGS)"
+	@echo "USER_FLAGS: $(USER_FLAGS)"
+	@echo "CC_OPTS: $(CC_OPTS)"
 
 
 # -----------------------------------------------------------------------------
 # Help
 # -----------------------------------------------------------------------------
 help:
-	@echo "NEORV32 Software Application Makefile"
+	@echo "NEORV32 Software Makefile"
 	@echo "Find more information at https://github.com/stnolting/neorv32"
 	@echo ""
 	@echo "Targets:"

sw/lib/source/neorv32_uart.c

@@ -284,21 +284,12 @@ void neorv32_uart_puts(neorv32_uart_t *UARTx, const char *s) {
 /**********************************************************************//**
  * Custom version of 'vprintf' printing to UART.
  *
+ * @warning: This functions only provides a minimal subset of the 'vprintf' formating features!
  * @note This function is blocking.
  *
  * @param[in,out] UARTx Hardware handle to UART register struct, #neorv32_uart_t.
  * @param[in] format Pointer to format string.
  * @param[in] args A value identifying a variable arguments list.
- *
- * <TABLE>
- * <TR><TD>%s</TD><TD>String (array of chars, zero-terminated)</TD></TR>
- * <TR><TD>%c</TD><TD>Single char</TD></TR>
- * <TR><TD>%d/%i</TD><TD>32-bit signed number, printed as decimal</TD></TR>
- * <TR><TD>%u</TD><TD>32-bit unsigned number, printed as decimal</TD></TR>
- * <TR><TD>%x</TD><TD>32-bit number, printed as 8-char hexadecimal - lower-case</TD></TR>
- * <TR><TD>%X</TD><TD>32-bit number, printed as 8-char hexadecimal - upper-case</TD></TR>
- * <TR><TD>%p</TD><TD>32-bit pointer, printed as 8-char hexadecimal - lower-case</TD></TR>
- * </TABLE>
  **************************************************************************/
 void neorv32_uart_vprintf(neorv32_uart_t *UARTx, const char *format, va_list args) {
 
@@ -344,11 +335,21 @@ void neorv32_uart_vprintf(neorv32_uart_t *UARTx, const char *format, va_list arg
           neorv32_uart_puts(UARTx, string_buf);
           break;
 
+//      case 'f': // floating point: print binary representation in hex
+//        union { double fp64; uint32_t u32[2]; } fp2hex;
+//        neorv32_uart_puts(UARTx, "FP64:0x");
+//        fp2hex.fp64 = va_arg(args, double); // C promotes float to double!
+//        __neorv32_uart_tohex(fp2hex.u32[0], string_buf);
+//        neorv32_uart_puts(UARTx, string_buf);
+//        __neorv32_uart_tohex(fp2hex.u32[1], string_buf);
+//        neorv32_uart_puts(UARTx, string_buf);
+//        break;
+
         case '%': // escaped percent sign
-          neorv32_uart_putc(UARTx, '%');
+          neorv32_uart_putc(UARTx, c);
           break;
 
-        default: // unsupported format
+        default: // unsupported formating character
           neorv32_uart_putc(UARTx, '%');
           neorv32_uart_putc(UARTx, c);
           break;
@@ -367,6 +368,7 @@ void neorv32_uart_vprintf(neorv32_uart_t *UARTx, const char *format, va_list arg
 /**********************************************************************//**
  * Custom version of 'printf' printing to UART.
  *
+ * @warning: This functions only provides a minimal subset of the 'printf' formating features!
  * @note This function is blocking.
  *
  * @param[in,out] UARTx Hardware handle to UART register struct, #neorv32_uart_t.