Multiplier Implementation
The Cortex-M0 and Cortex-M0+ CPU cores can be implemented with one of two hardware multiply options:
Fast : This allows the MULS instruction to execute in a single cycle
Small : An iterative multiplier that takes 32 cycles to execute a MULS instruction.
For most NXP MCU's which use these cores, the 'Fast' option is implemented. However the Cortex-M0 on LPC43xx and the Cortex-M0+ on LPC5410x implement the 'Small' option.
Changing compiler behavior for 'Slow' multiplier implementation
To multiply two integer variables the GCC compiler used by LPCXpresso IDE will always use a MULS instruction.
But for a multiply by a constant, the compiler can either use a sequence of add / subtract / shift operations, or the MULS instruction.
By default, the compiler assumes that a 'Fast' multiplier option is implemented by the target hardware, thus it will use a MULS instruction as this is assumed to be fastest and smallest. However this is not actually the case for Cortex-M0 on LPC43xx and the Cortex-M0+ on LPC5410x, and in some cases it may be preferable to generate sequence of add / subtract / shift operations in order to obtain better performance.
In order to allow this, LPCXpresso 7.6 introduced a new mechanism to allow the user to instruct GCC to generate add / subtract / shift operations.
To turn this on, go to
Project -> Properties -> C/C++ Build -> Settings -> Tool Settings …
… -> MCU C Compiler -> Architecture
and select the small multiplier version of Cortex-M0:
For example if this is done for the following simple function.
int mult (int a) {
return a * 42;
}
it will change the generated code (compiled -O1) from:
00000000 <mult>:
0:232a movs r3, #42; 0x2a
2:4358 muls r0, r3
4:4770 bx lr
6:46c0 nop ; (mov r8, r8)
to:
00000000 <mult>:
0:0043 lsls r3, r0, #1
2:1818 adds r0, r3, r0
4:00c3 lsls r3, r0, #3
6:1a18 subs r0, r3, r0
8:0040 lsls r0, r0, #1
a:4770 bx lr
Notes
- When deciding whether to change a MULS into multiple add/subtract/shift instructions, the compiler will not carry out the change if it would mean ending up with more than 5 instructions.
- MULS instructions will not be changed into multiple add/subtract/shift instructions when compiing -Os, as when this option is used code size is considered to be more important than performance.
