/// \brief Assign DWARF discriminators.
///
/// To assign discriminators, we examine the boundaries of every
/// basic block and its successors. Suppose there is a basic block B1
/// with successor B2. The last instruction I1 in B1 and the first
/// instruction I2 in B2 are located at the same file and line number.
/// This situation is illustrated in the following code snippet:
///
/// if (i < 10) x = i;
///
/// entry:
/// br i1 %cmp, label %if.then, label %if.end, !dbg !10
/// if.then:
/// %1 = load i32* %i.addr, align 4, !dbg !10
/// store i32 %1, i32* %x, align 4, !dbg !10
/// br label %if.end, !dbg !10
/// if.end:
/// ret void, !dbg !12
///
/// Notice how the branch instruction in block 'entry' and all the
/// instructions in block 'if.then' have the exact same debug location
/// information (!dbg !10).
///
/// To distinguish instructions in block 'entry' from instructions in
/// block 'if.then', we generate a new lexical block for all the
/// instruction in block 'if.then' that share the same file and line
/// location with the last instruction of block 'entry'.
///
/// This new lexical block will have the same location information as
/// the previous one, but with a new DWARF discriminator value.
///
/// One of the main uses of this discriminator value is in runtime
/// sample profilers. It allows the profiler to distinguish instructions
/// at location !dbg !10 that execute on different basic blocks. This is
/// important because while the predicate 'if (x < 10)' may have been
/// executed millions of times, the assignment 'x = i' may have only
/// executed a handful of times (meaning that the entry->if.then edge is
/// seldom taken).
///
/// If we did not have discriminator information, the profiler would
/// assign the same weight to both blocks 'entry' and 'if.then', which
/// in turn will make it conclude that the entry->if.then edge is very
/// hot.
///
/// To decide where to create new discriminator values, this function
/// traverses the CFG and examines instruction at basic block boundaries.
/// If the last instruction I1 of a block B1 is at the same file and line
/// location as instruction I2 of successor B2, then it creates a new
/// lexical block for I2 and all the instruction in B2 that share the same
/// file and line location as I2. This new lexical block will have a
/// different discriminator number than I1.
bool AddDiscriminators::runOnFunction(Function &F) {
// If the function has debug information, but the user has disabled
// discriminators, do nothing.
// Simlarly, if the function has no debug info, do nothing.
// Finally, if this module is built with dwarf versions earlier than 4,
// do nothing (discriminator support is a DWARF 4 feature).
if (NoDiscriminators ||
!hasDebugInfo(F) ||
F.getParent()->getDwarfVersion() < 4)
return false;
bool Changed = false;
Module *M = F.getParent();
LLVMContext &Ctx = M->getContext();
DIBuilder Builder(*M, /*AllowUnresolved*/ false);
// Traverse all the blocks looking for instructions in different
// blocks that are at the same file:line location.
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
BasicBlock *B = I;
TerminatorInst *Last = B->getTerminator();
DILocation LastDIL = Last->getDebugLoc().get();
if (!LastDIL)
continue;
for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) {
BasicBlock *Succ = Last->getSuccessor(I);
Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime();
DILocation FirstDIL = First->getDebugLoc().get();
if (!FirstDIL)
continue;
// If the first instruction (First) of Succ is at the same file
// location as B's last instruction (Last), add a new
// discriminator for First's location and all the instructions
// in Succ that share the same location with First.
if (!FirstDIL->canDiscriminate(*LastDIL)) {
// Create a new lexical scope and compute a new discriminator
// number for it.
StringRef Filename = FirstDIL->getFilename();
auto *Scope = FirstDIL->getScope();
auto *File = Builder.createFile(Filename, Scope->getDirectory());
// FIXME: Calculate the discriminator here, based on local information,
// and delete MDLocation::computeNewDiscriminator(). The current
// solution gives different results depending on other modules in the
// same context. All we really need is to discriminate between
// FirstDIL and LastDIL -- a local map would suffice.
unsigned Discriminator = FirstDIL->computeNewDiscriminator();
auto *NewScope =
Builder.createLexicalBlockFile(Scope, File, Discriminator);
auto *NewDIL =
MDLocation::get(Ctx, FirstDIL->getLine(), FirstDIL->getColumn(),
NewScope, FirstDIL->getInlinedAt());
DebugLoc newDebugLoc = NewDIL;
// Attach this new debug location to First and every
// instruction following First that shares the same location.
for (BasicBlock::iterator I1(*First), E1 = Succ->end(); I1 != E1;
++I1) {
if (I1->getDebugLoc().get() != FirstDIL)
break;
I1->setDebugLoc(newDebugLoc);
DEBUG(dbgs() << NewDIL->getFilename() << ":" << NewDIL->getLine()
<< ":" << NewDIL->getColumn() << ":"
<< NewDIL->getDiscriminator() << *I1 << "\n");
}
DEBUG(dbgs() << "\n");
Changed = true;
}
}
}
return Changed;
}
