[sancov] Generalize the code to get the previous instruction to multiple architectures

[george.karpenkov]

Summary

sancov subtracts one from the address to get the previous instruction, which makes sense on x86_64, but not on other platforms.
This change ensures that the offset is correct for different platforms.
The logic for computing the offset is copied from sanitizer_common.

[george.karpenkov]

@kcc together with https://reviews.llvm.org/D53040 it enables a bunch of previously disabled tests on ARM.

[morehouse]

LGTM

[Dor1s]

LGTM

llvm/tools/sancov/sancov.cpp

findSanitizerCovFunctions(const object::ObjectFile &O) {
 
   if (const auto *MO = dyn_cast<object::MachOObjectFile>(&O)) {
     findMachOIndirectCovFunctions(*MO, &Result);
   }
 
   return Result;
 }
 
+static uint64_t getPreviousInstructionPc(uint64_t PC,
+                                         Triple TheTriple) {
+  if (TheTriple.isARM()) {
+    return (PC - 3) & (~1);
+  } else if (TheTriple.isAArch64()) {
+    return PC - 4;
+  } else if (TheTriple.isMIPS()) {
+    return PC - 8;
+  } else {
+    return PC - 1;
+  }
+}
+
 // Locate addresses of all coverage points in a file. Coverage point
 // is defined as the 'address of instruction following __sanitizer_cov
 // call - 1'.
 static void getObjectCoveragePoints(const object::ObjectFile &O,
                                     std::set<uint64_t> *Addrs) {
   Triple TheTriple("unknown-unknown-unknown");
   TheTriple.setArch(Triple::ArchType(O.getArch()));
   auto TripleName = TheTriple.getTriple();
for (uint64_t Index = 0, Size = 0; Index < Section.getSize();
       if (!DisAsm->getInstruction(Inst, Size, Bytes.slice(Index),
                                   SectionAddr + Index, nulls(), nulls())) {
         if (Size == 0)
           Size = 1;
         continue;
       }
       uint64_t Addr = Index + SectionAddr;
       // Sanitizer coverage uses the address of the next instruction - 1.
-      uint64_t CovPoint = Addr + Size - 1;
+      uint64_t CovPoint = getPreviousInstructionPc(Addr + Size, TheTriple);
       uint64_t Target;
       if (MIA->isCall(Inst) &&
           MIA->evaluateBranch(Inst, SectionAddr + Index, Size, Target) &&
           SanCovAddrs.find(Target) != SanCovAddrs.end())
         Addrs->insert(CovPoint);
     }
   }
 }