Merge #147

147: Use the absolute addresses for calculation in entry assembly r=labbott a=labbott

Because RISC-V does not currently have full relocation support,
the addresses in the crt0 header are the exact addreses, not
offsets. This means we don't need to offset by the stack top again
to get the app heap start. Adjust the assembly to account for this.

Co-authored-by: Laura Abbott <laura@labbott.name>

src/entry_point/start_item_riscv32.rs

diff --git a/src/entry_point/start_item_riscv32.rs b/src/entry_point/start_item_riscv32.rs
index cabfd66..d2c5fd1 100644
--- a/src/entry_point/start_item_riscv32.rs
+++ b/src/entry_point/start_item_riscv32.rs
@@ -50,8 +50,7 @@
     // Otherwise after the first syscall (the memop to set the brk), the return
     // will use a stack that is outside of the process accessible memory.
     //
-    add t2, t0, t1          // t2 = stacktop + appdata_size
-    bgt t2, a3, skip_set_sp // Compare `app_heap_break` with new brk.
+    bgt t1, a3, skip_set_sp // Compare `app_heap_break` with new brk.
                                 // If our current `app_heap_break` is larger
                                 // then we need to move the stack pointer
                                 // before we call the `brk` syscall.
@@ -64,7 +63,7 @@
     // memop(0, stacktop + appdata_size);
     li  a0, 4               // a0 = 4   // memop syscall
     li  a1, 0               // a1 = 0
-    mv  a2, t2              // a2 = stacktop + appdata_size
+    mv  a2, t1              // a2 = appdata_size
     ecall                   // memop
     //
     // Debug support, tell the kernel the stack location
@@ -80,7 +79,7 @@
     // memop(11, stacktop + appdata_size);
     li  a0, 4               // a0 = 4   // memop syscall
     li  a1, 11              // a1 = 10
-    mv  a2, t2              // a2 = stacktop + appdata_size
+    mv  a2, t1              // a2 = appdata_size
     ecall                   // memop
     //
     // Setup initial stack pointer for normal execution