[MC] Fix undefined behavior in MCInstPrinter::formatHex

Passing INT64_MIN to MCInstPrinter::formatHex triggers undefined
behavior because the negation of -9223372036854775808 cannot be
represented in type 'int64_t' (aka 'long long'). This patch puts a
workaround in place to just print the hex value directly.

A possible alternative involves using a small helper functions that uses
(implementation) defined conversions to achieve the desirable value:

  static int64_t helper(int64_t V) {
    auto U = static_cast<uint64_t>(V);
    return V < 0 ? -U : U;
  }

The underlying problem is that MCInstPrinter::formatHex(int64_t) returns
a format_object<int64_t> and should really return a
format_object<uint64_t>. However, that's not possible because formatImm
needs to be able to print both as decimal (where a signed is required)
and hex (where we'd prefer to always have an unsigned).

  format_object<int64_t> formatImm(int64_t Value) const {
    return PrintImmHex ? formatHex(Value) : formatDec(Value);
  }

Differential revision: https://reviews.llvm.org/D67236

llvm-svn: 371159

lib/MC/MCInstPrinter.cpp

@@ -83,24 +83,25 @@ format_object<int64_t> MCInstPrinter::formatDec(int64_t Value) const {
 }
 
 format_object<int64_t> MCInstPrinter::formatHex(int64_t Value) const {
-  switch(PrintHexStyle) {
+  switch (PrintHexStyle) {
   case HexStyle::C:
-    if (Value < 0)
+    if (Value < 0) {
+      if (Value == std::numeric_limits<int64_t>::min())
+        return format<int64_t>("-0x8000000000000000", Value);
       return format("-0x%" PRIx64, -Value);
-    else
-      return format("0x%" PRIx64, Value);
+    }
+    return format("0x%" PRIx64, Value);
   case HexStyle::Asm:
     if (Value < 0) {
-      if (needsLeadingZero((uint64_t)(-Value)))
+      if (Value == std::numeric_limits<int64_t>::min())
+        return format<int64_t>("-8000000000000000h", Value);
+      if (needsLeadingZero(-(uint64_t)(Value)))
         return format("-0%" PRIx64 "h", -Value);
-      else
-        return format("-%" PRIx64 "h", -Value);
-    } else {
-      if (needsLeadingZero((uint64_t)(Value)))
-        return format("0%" PRIx64 "h", Value);
-      else
-        return format("%" PRIx64 "h", Value);
+      return format("-%" PRIx64 "h", -Value);
     }
+    if (needsLeadingZero((uint64_t)(Value)))
+      return format("0%" PRIx64 "h", Value);
+    return format("%" PRIx64 "h", Value);
   }
   llvm_unreachable("unsupported print style");
 }

unittests/MC/CMakeLists.txt

@@ -8,6 +8,7 @@ set(LLVM_LINK_COMPONENTS
 add_llvm_unittest(MCTests
   Disassembler.cpp
   DwarfLineTables.cpp
+  MCInstPrinter.cpp
   StringTableBuilderTest.cpp
   TargetRegistry.cpp
   )

unittests/MC/MCInstPrinter.cpp

@@ -0,0 +1,68 @@
+//===- llvm/unittest/MC/MCInstPrinter.cpp ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/MC/MCInstPrinter.h"
+#include "llvm/MC/MCAsmInfo.h"
+#include "llvm/MC/MCInstrInfo.h"
+#include "llvm/Support/TargetRegistry.h"
+#include "llvm/Support/TargetSelect.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetOptions.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+class MCInstPrinterTest : public ::testing::Test {
+public:
+  std::unique_ptr<MCRegisterInfo> MRI;
+  std::unique_ptr<MCAsmInfo> MAI;
+  std::unique_ptr<const MCInstrInfo> MII;
+  std::unique_ptr<MCInstPrinter> Printer;
+
+  MCInstPrinterTest() {
+    llvm::InitializeAllTargetInfos();
+    llvm::InitializeAllTargetMCs();
+
+    std::string TripleName = "x86_64-pc-linux";
+    std::string ErrorStr;
+
+    const Target *TheTarget =
+        TargetRegistry::lookupTarget(TripleName, ErrorStr);
+
+    // If we didn't build x86, do not run the test.
+    if (!TheTarget)
+      return;
+
+    MRI.reset(TheTarget->createMCRegInfo(TripleName));
+    MAI.reset(TheTarget->createMCAsmInfo(*MRI, TripleName));
+    MII.reset(TheTarget->createMCInstrInfo());
+    Printer.reset(TheTarget->createMCInstPrinter(
+        Triple(TripleName), MAI->getAssemblerDialect(), *MAI, *MII, *MRI));
+  }
+
+  template <typename T> std::string formatHex(T i) {
+    std::string Buffer;
+    raw_string_ostream OS(Buffer);
+    OS << Printer->formatHex(i);
+    OS.flush();
+    return Buffer;
+  }
+};
+} // namespace
+
+TEST_F(MCInstPrinterTest, formatHex) {
+  if (!Printer)
+    return;
+
+  EXPECT_EQ("0x1", formatHex<int64_t>(1));
+  EXPECT_EQ("0x7fffffffffffffff",
+            formatHex(std::numeric_limits<int64_t>::max()));
+  EXPECT_EQ("-0x8000000000000000",
+            formatHex(std::numeric_limits<int64_t>::min()));
+}