[X86][MC] no error diagnostic for out-of-range jrcxz/jecxz/jcxz

Fix for PR24072:

X86 instructions jrcxz/jecxz/jcxz performs short jumps if rcx/ecx/cx register is 0
The maximum relative offset for a forward short jump is 127 Bytes (0x7F).
The maximum relative offset for a backward short jump is 128 Bytes (0x80).

Gnu assembler warns when the distance of the jump exceeds the maximum but llvm-as does not.

Patch by Konstantin Belochapka and Alexey Lapshin

Differential Revision: https://reviews.llvm.org/D70652

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -12,6 +12,8 @@
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/BinaryFormat/MachO.h"
 #include "llvm/MC/MCAsmBackend.h"
+#include "llvm/MC/MCAssembler.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDwarf.h"
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCExpr.h"
@@ -22,6 +24,7 @@
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/MC/MCSubtargetInfo.h"
+#include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
@@ -114,12 +117,24 @@ class X86AsmBackend : public MCAsmBackend {
 
     assert(Fixup.getOffset() + Size <= Data.size() && "Invalid fixup offset!");
 
-    // Check that uppper bits are either all zeros or all ones.
-    // Specifically ignore overflow/underflow as long as the leakage is
-    // limited to the lower bits. This is to remain compatible with
-    // other assemblers.
-    assert((Size == 0 || isIntN(Size * 8 + 1, Value)) &&
-           "Value does not fit in the Fixup field");
+    int64_t SignedValue = static_cast<int64_t>(Value);
+    if ((Target.isAbsolute() || IsResolved) &&
+        getFixupKindInfo(Fixup.getKind()).Flags &
+            MCFixupKindInfo::FKF_IsPCRel) {
+      // check that PC relative fixup fits into the fixup size.
+      if (Size > 0 && !isIntN(Size * 8, SignedValue))
+        Asm.getContext().reportError(
+            Fixup.getLoc(), "value of " + Twine(SignedValue) +
+                                " is too large for field of " + Twine(Size) +
+                                ((Size == 1) ? " byte." : " bytes."));
+    } else {
+      // Check that uppper bits are either all zeros or all ones.
+      // Specifically ignore overflow/underflow as long as the leakage is
+      // limited to the lower bits. This is to remain compatible with
+      // other assemblers.
+      assert((Size == 0 || isIntN(Size * 8 + 1, SignedValue)) &&
+             "Value does not fit in the Fixup field");
+    }
 
     for (unsigned i = 0; i != Size; ++i)
       Data[Fixup.getOffset() + i] = uint8_t(Value >> (i * 8));

llvm/test/MC/MachO/reloc.s

@@ -37,7 +37,7 @@ L0:
         .text
 _f0:
 L1:
-        jmp	0xbabecafe
+        jmp	0x7abecafe
         jmp L0
         jmp L1
         ret

llvm/test/MC/X86/x86-jcxz-loop-fixup.s

@@ -0,0 +1,26 @@
+# RUN: not llvm-mc -filetype=obj -triple=x86_64-linux-gnu %s 2>&1 | FileCheck %s
+
+       .balign 128 
+label00:
+// CHECK: value of 253 is too large for field of 1 byte.
+  jecxz   label01
+// CHECK: value of 251 is too large for field of 1 byte.
+  jrcxz   label01
+// CHECK: value of 249 is too large for field of 1 byte.
+  loop  label01
+// CHECK: value of 247 is too large for field of 1 byte. 
+  loope  label01
+// CHECK: value of 245 is too large for field of 1 byte.
+  loopne  label01
+        .balign 256 
+label01:
+// CHECK: value of -259 is too large for field of 1 byte.
+  jecxz   label00
+// CHECK: value of -261 is too large for field of 1 byte.
+  jrcxz   label00
+// CHECK: value of -263 is too large for field of 1 byte.
+  loop  label00
+// CHECK: value of -265 is too large for field of 1 byte.
+  loope  label00
+// CHECK: value of -267 is too large for field of 1 byte.
+  loopne  label00