[PATCH] Disable branch folding with MemorySanitizer

[David Blaikie]

Do we have precedence for this kind of change (where sanitizers affect
optimizations in arbitrary internal ways - not simply by enabling/disabling
certain passes)? If not, does this need some deeper discussion about
alternatives (is it important that we be able to produce equivalent code
without the sanitizers enabled?)?


On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov <eugenis at google.com>wrote:

> 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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