On Sun, 14 Jan 2024, amindfv--- via Haskell-Cafe wrote:

I have a function that's called many (many) times, which involves a saturating addition (addition without overflow).

In other words, I'd like ((254 :: Word8) + 10) to equal 255 (which is maxBound::Word8), not 8 (which is what you get with overflow).

My current code, without particular regard to performance, is simply:

satAdd :: Word8 -> Word8 -> Word8 satAdd x y = let m = x + y in if m < x then 255 else m

This seems like the type of code somebody's already worked on the performance for, though I can't find much Haskell-specific in searches online.

LLVM is pretty good at optimizing such code snippets. You may first watch what code it generates. I guess you do not have to apply manual optimizations. But you might be curious how LLVM translates your code. The CPU sets a flag when it encounters a carry (aka unsigned overflow). Then it has an instruction for doing a conditional move. That is, even without special saturated arithmetic instructions, unsigned saturated addition can be done in two or three basic instructions. LLVM also has a saturating add intrinsic: https://llvm.org/docs/LangRef.html#saturation-arithmetic-intrinsics x86 provides saturating addition in its SSE and AVX subsystems. They even allow you to perform up to 64 byte additions in one instruction. I have looked up that x86 has PADDUSB instruction. Ok, lets watch LLVM: $ cat saturated-add.ll target triple = "x86_64-unknown-linux-gnu" define i8 @satAdd(i8 %x, i8 %y) { %m = add i8 %x, %y %carry = icmp ult i8 %m, %x %satSum = select i1 %carry, i8 255, i8 %m ret i8 %satSum } $ llc-17 <saturated-add.ll ... satAdd: # @satAdd .cfi_startproc # %bb.0: addb %sil, %dil movzbl %dil, %ecx movl $255, %eax cmovael %ecx, %eax retq ... Now with optimizations: $ opt-17 -O2 <saturated-add.ll | llvm-dis-17 ... define i8 @satAdd(i8 %x, i8 %y) local_unnamed_addr #0 { %satSum = tail call i8 @llvm.uadd.sat.i8(i8 %x, i8 %y) ret i8 %satSum } ... declare i8 @llvm.uadd.sat.i8(i8, i8) #1 Thus, it has detected your manual saturated add implementation. To what instruction will it be translated? $ opt-17 -O2 <saturated-add.ll | llc-17 ... satAdd: # @satAdd # %bb.0: addb %sil, %dil movzbl %dil, %ecx movl $255, %eax cmovael %ecx, %eax retq Sadly, it remains the same. No special instructions.