[llvm] [AArch64][SVE] Tweak how SVE CFI expressions are emitted (PR #151677)

[Benjamin Maxwell via llvm-commits]

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@@ -5861,33 +5861,41 @@ void AArch64InstrInfo::decomposeStackOffsetForFrameOffsets(
   }
 }
 
-// Convenience function to create a DWARF expression for
-//   Expr + NumBytes + NumVGScaledBytes * AArch64::VG
-static void appendVGScaledOffsetExpr(SmallVectorImpl<char> &Expr, int NumBytes,
-                                     int NumVGScaledBytes, unsigned VG,
-                                     llvm::raw_string_ostream &Comment) {
-  uint8_t buffer[16];
-
-  if (NumBytes) {
+// Convenience function to create a DWARF expression for `Op` Value.
+// This helper emits compact sequences for common cases.
+static void appendConstantExpr(SmallVectorImpl<char> &Expr, int64_t Value,
+                               uint8_t Op) {
+  // + -constant (<= 31)
+  if (Op == dwarf::DW_OP_plus && Value < 0 && -Value <= 31) {
+    Expr.push_back(dwarf::DW_OP_lit0 - Value);
+    Op = dwarf::DW_OP_minus;
+  } else if (Value >= 0 && Value <= 31) {
+    // `Op` literal value 0 to 31
+    Expr.push_back(dwarf::DW_OP_lit0 + Value);
+  } else {
+    // `Op` constant
     Expr.push_back(dwarf::DW_OP_consts);
-    Expr.append(buffer, buffer + encodeSLEB128(NumBytes, buffer));
-    Expr.push_back((uint8_t)dwarf::DW_OP_plus);
-    Comment << (NumBytes < 0 ? " - " : " + ") << std::abs(NumBytes);
+    appendLEB128<LEB128Sign::Signed>(Expr, Value);
   }
+  return Expr.push_back(Op);
+}
 
-  if (NumVGScaledBytes) {
-    Expr.push_back((uint8_t)dwarf::DW_OP_consts);
-    Expr.append(buffer, buffer + encodeSLEB128(NumVGScaledBytes, buffer));
-
-    Expr.push_back((uint8_t)dwarf::DW_OP_bregx);
-    Expr.append(buffer, buffer + encodeULEB128(VG, buffer));
-    Expr.push_back(0);
-
-    Expr.push_back((uint8_t)dwarf::DW_OP_mul);
-    Expr.push_back((uint8_t)dwarf::DW_OP_plus);
+// Convenience function to create a DWARF expression for VG
+static void appendReadVGRegExpr(SmallVectorImpl<char> &Expr,
+                                unsigned VGRegNum) {
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MacDue wrote:

In my next patch, I will add `appendLoadIncomingVGExpr`. I think the more explicit name here makes the distinction more apparent. 

https://github.com/llvm/llvm-project/pull/151677