Salvage debug info from instructions about to be deleted

[Reapplies r297971 and punting on finding a better API for findDbgValues()]

This patch improves debug info quality in InstCombine by looking at
values that are about to be deleted, checking whether there are any
dbg.value instrinsics referring to them, and potentially encoding the
semantics of the deleted instruction into the dbg.value's
DIExpression.

In the example in the testcase (which was extracted from XNU) there is a sequence of

 %4 = load %struct.entry*, %struct.entry** %next2, align 8, !dbg !41
 %5 = bitcast %struct.entry* %4 to i8*, !dbg !42
 %add.ptr4 = getelementptr inbounds i8, i8* %5, i64 -8, !dbg !43
 %6 = bitcast i8* %add.ptr4 to %struct.entry*, !dbg !44
 call void @llvm.dbg.value(metadata %struct.entry* %6, i64 0, metadata !20, metadata !21), !dbg 34

When these instructions are eliminated by instcombine one after
another, we can still salvage the otherwise dead debug info:

- Bitcasts have no effect, so have the dbg.value point to operand(0)
- Loads can be expressed via a DW_OP_deref
- Constant gep instructions can be replaced by DWARF expression arithmetic

The API introduced by this patch is not specific to instcombine and
can be useful in other places, too.

rdar://problem/30725338

Differential Revision: https://reviews.llvm.org/D30919

llvm-svn: 297994

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 0d76cdf371e..0a3382e1903 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -28,6 +28,9 @@
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Pass.h"
 #include "llvm/Transforms/InstCombine/InstCombineWorklist.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/Dwarf.h"
+#include "llvm/IR/DIBuilder.h"
 
 #define DEBUG_TYPE "instcombine"
 
@@ -470,8 +473,9 @@ public:
   /// methods should return the value returned by this function.
   Instruction *eraseInstFromFunction(Instruction &I) {
     DEBUG(dbgs() << "IC: ERASE " << I << '\n');
-
     assert(I.use_empty() && "Cannot erase instruction that is used!");
+    salvageDebugInfo(I);
+
     // Make sure that we reprocess all operands now that we reduced their
     // use counts.
     if (I.getNumOperands() < 8) {