[PATCH] Disable branch folding with MemorySanitizer

[Evgeniy Stepanov]

Branch folding optimization often leads to confusing MSan reports due to lost debug info.
For example,
1: if (x < 0)
2:   if (y < 0)
3:    do_something();
is transformed into something like
  %0 = and i32 %y, %x
  %1 = icmp slt i32 %0, 0
  br i1 %1, label %if.then2, label %if.end3
where all 3 instructions are associated with line 1.

This patch disables folding of conditional branches in functions with sanitize_memory attribute.

http://llvm-reviews.chandlerc.com/D2214

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/branch-fold-msan.ll

Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1967,6 +1967,13 @@
 bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
   BasicBlock *BB = BI->getParent();
 
+  // This optimization results in confusing MemorySanitizer reports. Use of
+  // uninitialized value in this branch instruction is reported with the
+  // predecessor's debug location.
+  if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) &&
+      BI->isConditional())
+    return false;
+
   Instruction *Cond = 0;
   if (BI->isConditional())
     Cond = dyn_cast<Instruction>(BI->getCondition());
Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll
===================================================================
--- test/Transforms/SimplifyCFG/branch-fold-msan.ll
+++ test/Transforms/SimplifyCFG/branch-fold-msan.ll
@@ -0,0 +1,29 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+declare void @callee()
+
+; Test that conditional branches are not folded with sanitize_memory.
+define void @caller(i32 %x, i32 %y) sanitize_memory {
+; CHECK: define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]])
+; CHECK: icmp slt i32 {{.*}}[[X]]
+; CHECK: icmp slt i32 {{.*}}[[Y]]
+; CHECK: ret void
+
+entry:
+  %cmp = icmp slt i32 %x, 0
+  br i1 %cmp, label %if.then, label %if.end3
+
+if.then:                                          ; preds = %entry
+  %cmp1 = icmp slt i32 %y, 0
+  br i1 %cmp1, label %if.then2, label %if.end
+
+if.then2:                                         ; preds = %if.then
+  call void @callee()
+  br label %if.end
+
+if.end:                                           ; preds = %if.then2, %if.then
+  br label %if.end3
+
+if.end3:                                          ; preds = %if.end, %entry
+  ret void
+}
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