Slighly clean up symbol loading error.

The symbol not found error that is triggered
during lazy-loading was a bit chaotic before.

This reformats it a bit to:

```
ghc-stage2.exe:  | E:\...\libLLVMSupport.a: unknown symbol `_ZN4llvm5APIntC1Ejyb'
ghc-stage2.exe:  | E:\...\libLLVMCore.a: unknown symbol `_ZN4llvm5APInt14AssignSlowCaseERKS0_'
ghc-stage2.exe:  | E:\...\libLLVMCore.a: unknown symbol `_ZN4llvm13ConstantRangeC1ENS_5APIntES1_'
ghc-stage2.exe:  | E:\...\libLLVMCore.a: unknown symbol `_ZN4llvm14FoldingSetImplC2Ej'
ghc-stage2.exe:  | E:\...\libLLVMCore.a: unknown symbol `_ZN4llvm15LLVMContextImplD1Ev'
ghc-stage2.exe:  | E:\...\libLLVMLTO.a: unknown symbol `_ZN4llvm11LLVMContextD1Ev'
ghc-stage2.exe:  | E:\...\libLLVMCore.a: unknown symbol `_ZNK4llvm5Value10getContextEv'
ghc-stage2.exe: ^^ Could not load 'LLVMIsMultithreaded', dependency unresolved.
See top entry above.
```

I have also thought about also showing the demangled names, as it may
be useful for the end user.

`libgcc` seems to provide a method for this so we wouldn't need any
extra dependency.

Any thoughts on this or would it not be useful?

Reviewers: austin, erikd, simonmar, bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D3027

GHC Trac Issues: #13093, #13113

1 files changed, 5 insertions, 2 deletions

diff --git a/rts/Linker.c b/rts/Linker.c
index 9462bdbbcc..8945a96d1e 100644
--- a/rts/Linker.c
+++ b/rts/Linker.c
@@ -870,12 +870,11 @@ SymbolAddr* loadSymbol(SymbolName *lbl, RtsSymbolInfo *pinfo) {
 
     /* Symbol can be found during linking, but hasn't been relocated. Do so now.
         See Note [runtime-linker-phases] */
-    if (oc && oc->status == OBJECT_LOADED) {
+    if (oc && lbl && oc->status == OBJECT_LOADED) {
         oc->status = OBJECT_NEEDED;
         IF_DEBUG(linker, debugBelch("lookupSymbol: on-demand loading symbol '%s'\n", lbl));
         int r = ocTryLoad(oc);
         if (!r) {
-            errorBelch("Could not on-demand load symbol '%s'\n", lbl);
             return NULL;
         }
 
@@ -893,6 +892,10 @@ SymbolAddr* lookupSymbol( SymbolName* lbl )
 {
     ACQUIRE_LOCK(&linker_mutex);
     SymbolAddr* r = lookupSymbol_(lbl);
+    if (!r) {
+        errorBelch("^^ Could not load '%s', dependency unresolved. See top entry above.\n", lbl);
+        fflush(stderr);
+    }
     RELEASE_LOCK(&linker_mutex);
     return r;
 }