Fix signed integer overflow

Author: antoyo

.github/workflows/release.yml

@@ -104,7 +104,8 @@ jobs:
 
     - name: Run tests
       run: |
-        ./test.sh --release --clean --release-sysroot --build-sysroot --mini-tests --std-tests # --test-libcore # FIXME(antoyo): libcore tests fail.
+        ./test.sh --release --clean --release-sysroot --build-sysroot --mini-tests --std-tests --test-libcore
+        # TODO(antoyo): also run the rustc's test suite.
 
     - name: Run stdarch tests
       run: |

src/base.rs

@@ -124,6 +124,8 @@ pub fn compile_codegen_unit<'tcx>(tcx: TyCtxt<'tcx>, cgu_name: Symbol, supports_
         context.add_command_line_option("-fno-semantic-interposition");
         // NOTE: Rust relies on LLVM not doing TBAA (https://github.com/rust-lang/unsafe-code-guidelines/issues/292).
         context.add_command_line_option("-fno-strict-aliasing");
+        // NOTE: Rust relies on LLVM doing wrapping on overflow.
+        context.add_command_line_option("-fwrapv");
 
         if tcx.sess.opts.unstable_opts.function_sections.unwrap_or(tcx.sess.target.function_sections) {
             context.add_command_line_option("-ffunction-sections");