[llvm] [ConstraintElim] Decompose shl nsw for signed predicates (PR #76961)

[Florian Hahn via llvm-commits]

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@@ -517,6 +517,17 @@ static Decomposition decompose(Value *V,
       return Result;
     }
 
+    // shl nsw x, shift is mul nsw x, (1<<shift),
+    // with the exception of shift == bw-1.
+    if (match(V, m_NSWShl(m_Value(Op0), m_ConstantInt(CI)))) {
+      uint64_t Shift = CI->getValue().getLimitedValue();
+      if (Shift < Ty->getIntegerBitWidth() - 1 && Shift < 64) {
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fhahn wrote:

Looks like there's no test case for `Shift >= 64` at the moment. IIUC this is to avoid overflows when doing `1 << Shift`? We won't reach this code if the types are wider than 64 bit and if the type is <= i64 then shifts >= 64 will produce poison. Might be helpful for future readers to document the condition

https://github.com/llvm/llvm-project/pull/76961