Debug Rcpp with ldbb: variables not available

Question

I'm making a package with RcppArmadillo under the latest version of Rstudio (1.3.959) and R (4.0.0).

Consider the following minimal example, which create the package anRpackage:

library(Rcpp)
library(RcppArmadillo)
RcppArmadillo.package.skeleton()

Setting the working directory in this package, I load it by:

library(devtools)
devtools::load_all()

Then the basic function rcpparma_hello_world() can be executed:

rcpparma_hello_world()

     [,1] [,2] [,3]
[1,]    7    0    0
[2,]    0    7    0
[3,]    0    0    7

• I want now to debug the code with lldb via the Terminal command R -d lldb in order to check the variable values during the execution via the command frame variable.

R -d lldb
(lldb) breakpoint set --name rcpparma_hello_world()
(lldb) run
> library(devtools)
> devtools::load_all()
> rcpparma_hello_world()

anRpackage.so was compiled with optimization - stepping may behave oddly; variables may not be available.

Indeed, the problem is that most of variables are not available:

> next
> frame variable

(arma::mat) m1 = <no location, value may have been optimized out>
(arma::mat) m2 = <no location, value may have been optimized out>

• I read in the previous link the following recommandations:

you may consider producing so-called ‘debug’ builds, with optimization toned down, when attempting to debug these kinds of issues as well. For building R packages, this effectively amounts to something like CXXFLAGS=-g -O0 in your ~/.R/Makevars file)

• I have tried it but it seems to have no effect under the compilation flags used since I have a local file Makevars in my package to specify the way of compiling with RcppArdimillo, whose flags are:

CXX_STD = CXX11
PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS) 
PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS) $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)

Then, the command devtools::load_all() produces the line-command compilation given below, in which there is indeed -g -O2 making some code optimization:

clang++ -mmacosx-version-min=10.13 -std=gnu++11 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I'/Library/Frameworks/R.framework/Versions/4.0/Resources/library/Rcpp/include' -I'/Library/Frameworks/R.framework/Versions/4.0/Resources/library/RcppArmadillo/include' -I/usr/local/include    -fPIC  -Wall -g -O2  -c rcpparma_hello_world.cpp -o rcpparma_hello_world.o

Question: how can I have access to the variables in debugging mode?

Thanks