What's the meaning of W suffix for thumb-2 instruction?

Question

There is a w suffix for thumb-2 instruction as below, how does it change the semantic of the instruction without it? The search result is very noisy and I didn't get the answer.

addw r0, r1, #0

Answer 1

Simply enough, W means "wide". It is the 32-bit version of the instruction, whereas most Thumb instructions are 16 bits wide. The wide instructions often have bigger immediates or can address more registers.

Edit: Some of the comments seem confused about the difference between addw and add.w. The only essential difference is how the immediate is encoded.

add.w:  imm32 = ThumbExpandImm(i:imm3:imm8);
addw:   imm32 = ZeroExtend(i:imm3:imm8, 32);

Answer 2

I see ADDW in Cortex-M3 TRM Table 2-5

Data operations with large immediate
ADDW and SUBW have a 12-bit immediate. This means they can replace many from memory literal loads.

It is also mentioned in Quick Reference

add wide T2 ADD Rd, Rn, #<imm12>

Looks like the assembler would recognize the immediate constant <= 12 bits, and do the needful.

In the context where you see it, it is an ordinary "add".