fix: log store only if it actually happened

Since 92e4f02 moved logging logic into store_slow_path function it has
been logging stores even if actually_store parameter is false. Because
of that logging is broken for all atomic instructions. Function "amo" calls
store_slow_path with nullptr argument and actually_store equal to false
while callee uses reg_from_bytes independently from actually_store value
All of that causes dereferencing of nullptr. This commit logs memory
access only if it actually happened

ci-tests/atomics.c

@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <stdatomic.h>
+
+atomic_int acnt = 0;
+atomic_int bcnt = 0;
+
+int foo() {
+  for(int n = 0; n < 1000; ++n) {
+    ++acnt;
+    if(acnt % 10 == 0)
+      ++bcnt;
+  }
+  return acnt;
+}
+
+int main(void) {
+  int acnt = foo();
+  printf("First atomic counter is %u, second is %u\n", acnt, bcnt);
+  return 0;
+}

ci-tests/create-ci-binary-tarball

@@ -20,10 +20,16 @@ mkdir -p build/dummycsr && cd "$_"
 riscv64-unknown-elf-gcc -O2 -o customcsr `git rev-parse --show-toplevel`/ci-tests/customcsr.c
 cd -
 
+mkdir -p build/atomics && cd "$_"
+riscv64-unknown-elf-gcc -O2 -o atomics `git rev-parse --show-toplevel`/ci-tests/atomics.c
+cd -
+
+
 mv build/pk/pk .
 mv build/hello/hello .
 mv build/dummy-slliuw/dummy-slliuw .
 mv build/dummycsr/customcsr .
-tar -cf spike-ci.tar pk hello dummy-slliuw customcsr
+mv build/atomics/atomics .
+tar -cf spike-ci.tar pk hello dummy-slliuw customcsr atomics
 
-rm pk hello dummy-slliuw customcsr
+rm pk hello dummy-slliuw customcsr atomics

ci-tests/test-spike

@@ -11,6 +11,7 @@ cd run
 wget https://github.com/riscv-software-src/riscv-isa-sim/releases/download/dummy-tag-for-ci-storage/spike-ci.tar
 tar xf spike-ci.tar
 time ../install/bin/spike --isa=rv64gc pk hello | grep "Hello, world!  Pi is approximately 3.141588."
+../install/bin/spike --log-commits --isa=rv64gc pk atomics | grep "First atomic counter is 1000, second is 100"
 
 # check that including sim.h in an external project works
 g++ -std=c++2a -I../install/include -L../install/lib $DIR/testlib.cc -lriscv -o test-libriscv

riscv/mmu.cc

@@ -369,7 +369,7 @@ void mmu_t::store_slow_path(reg_t original_addr, reg_t len, const uint8_t* bytes
     store_slow_path_intrapage(len, bytes, access_info, actually_store);
   }
 
-  if (proc && unlikely(proc->get_log_commits_enabled()))
+  if (actually_store && proc && unlikely(proc->get_log_commits_enabled()))
     proc->state.log_mem_write.push_back(std::make_tuple(original_addr, reg_from_bytes(len, bytes), len));
 }