| Issue |
169763
|
| Summary |
[InstCombine] Fold icmp ult (uadd.sat X, C), C2 to icmp ult X, (C2 - C)
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ParkHanbum
|
Description
I would like to propose a transformation that simplifies a check on uadd.sat result into a simple range check on the input, when the operands are constants. If we check whether a saturated unsigned addition is strictly less than a constant C2, and we know that C2 > C, we can elide the saturation logic entirely.
Pattern:
uadd.sat(X, C) < C2
Current Transform:
uadd_sat(X, Y) pred Z into
(-1 pred Z) == true => (X >= ~Y) || ((X + Y) pred Z)
(-1 pred Z) == false => (X < ~Y) && ((X + Y) pred Z)
Preconditions:
C and C2 are constants.
C2 u> C (Unsigned comparison).
SIGN_MAX u< C2 is false.
Reasoning:
(-1 u< C2) == false => (X u< ~C) && ((X +C) u< C2)
(X u< ~C) && ((X +C) u< C2) -> ((X + C) u< -1) && ((X + C) u< C2)
; Since C2 <= -1 (Unsigned Max) is always true,
; the condition ((X + C) u< C2) implies ((X + C) u< -1).
; Therefore, the intersection is simply:
=> (X + C) u< C2
=> X u< C2 - C
Alive Proof : https://alive2.llvm.org/ce/z/czW5Pj
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