[llvm] [DAG] Support saturated truncate (PR #99418)

[via llvm-commits]

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@@ -5343,9 +5343,13 @@ def VImm8000: PatLeaf<(AArch64mvni_msl (i32 127), (i32 264))>;
 // trunc(umin(X, 255)) -> UQXTRN v8i8
 def : Pat<(v8i8 (trunc (umin (v8i16 V128:$Vn), (v8i16 VImmFF)))),
           (UQXTNv8i8 V128:$Vn)>;
+def : Pat<(v8i8 (truncusat (v8i16 V128:$Vn))),
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ParkHanbum wrote:

@topperc I tested this by implementing additional code to support `fp_to_[su]int_sat`. As a result, if `LegalOperations` is unchecked, the associated Pattern can be deleted. 

Should we continue to check `LegalOperations`? I can not sure because I'm not a veteran. 


https://github.com/llvm/llvm-project/pull/99418