IRToASTVisitor: sign-extend from the source width #345

Reproduced with Rellic v0.2.3 (0cccc98a78d985bd0d1343f82b123dda03f71fcb), LLVM 15. The corresponding lowering remains in current master (370abaad86ae5adad82f85b81aff14557462b7f2); current-master behavior has only been source-inspected.

LLVM sext widens an unsigned C source directly to the signed destination, losing the source sign bit.

Save as repro.ll:

define i64 @extend32(i32 %source) {
  %result = sext i32 %source to i64
  ret i64 %result
}
define i64 @extend1(i1 %source) {
  %result = sext i1 %source to i64
  ret i64 %result
}

rellic-decomp --input repro.ll --output repro.c

For extend32, the emitted widening is equivalent to (long long)source with an unsigned 32-bit source. 0xffffffff consequently becomes 4294967295, whereas LLVM requires -1.

For sext i1, true must become all-one bits (-1), rather than integer 1.

Interpret the source at its signed LLVM width before widening, including the special one-bit case. Casting only the destination to a signed type is insufficient.