Missed optimization: remap `sadd.sat(x, C)` constant compares to input-side checks

[ParkHanbum]

Summary

LLVM appears to miss folds for constant comparisons against signed saturating
add with a constant RHS:

%sat = call iN @llvm.sadd.sat.iN(iN %x, iN C)
%cmp = icmp pred iN %sat, K

Because signed saturating add by a constant is monotonic, many constant
comparisons can be remapped to direct threshold compares or small range checks
on %x.

Examples

sadd.sat(x, -1) > -1  ->  x > 0
sadd.sat(x,  8) <  0  ->  x < -8
sadd.sat(x,  4) >  0  ->  x > -4
sadd.sat(x, -1) u> 15 ->  x < 1 || x > 16

The last case is a corpus-backed unsigned comparison against the signed
saturating-add result; it becomes a range-complement check rather than a single
signed threshold compare.

Minimal IR

declare i32 @llvm.sadd.sat.i32(i32, i32)
declare i64 @llvm.sadd.sat.i64(i64, i64)

define i1 @current_sgt(i32 %x) {
entry:
  %sat = call i32 @llvm.sadd.sat.i32(i32 %x, i32 -1)
  %cmp = icmp sgt i32 %sat, -1
  ret i1 %cmp
}

define i1 @folded_sgt(i32 %x) {
entry:
  %cmp = icmp sgt i32 %x, 0
  ret i1 %cmp
}

define i1 @current_slt(i64 %x) {
entry:
  %sat = call i64 @llvm.sadd.sat.i64(i64 %x, i64 8)
  %cmp = icmp slt i64 %sat, 0
  ret i1 %cmp
}

define i1 @folded_slt(i64 %x) {
entry:
  %cmp = icmp slt i64 %x, -8
  ret i1 %cmp
}

The local POC also includes the ugt range-complement case.

Corpus Evidence

Full optimized corpus scan found 207 reviewed true positives across 6 files and
4 projects.

Project distribution:

• yara-x-rs: 176
• llvm: 27
• icu: 3
• zed-rs: 1

Predicate distribution:

• slt: 189
• sgt: 17
• ugt: 1

Saturating-add constant distribution:

• 2: 112
• 8: 32
• 14: 32
• -1: 23
• 1: 6
• -2: 1
• 4: 1

Representative locations:

Project	File	Line	Predicate	C
icu	bench/icu/optimized/number_decimalquantity.ll	5687	sgt	-1
icu	bench/icu/optimized/number_decimalquantity.ll	5701	ugt	-1
llvm	bench/llvm/optimized/AArch64TargetTransformInfo.ll	5689	sgt	4
llvm	bench/llvm/optimized/RISCVTargetTransformInfo.ll	3730	slt	1
yara-x-rs	bench/yara-x-rs/optimized/98ju2vcu3mcgze6k61u00b6zf.ll	28227	slt	8

Suggested Fold Scope

Initial scope:

• signed saturating add intrinsic;
• constant RHS C;
• constant compare threshold K;
• predicates represented by the corpus: slt, sgt, and range-derived ugt.

Non-Goals

This draft does not claim unsigned saturating-add folds, variable C, variable
compare thresholds, or an LLVM implementation strategy.