[MCExpr] avoid UB via negation of INT_MIN

[spatel]

Summary

I accidentally exposed a bug in MCExpr::evaluateAsRelocatableImpl() with the test file added in:
http://reviews.llvm.org/rL269977

I don't know anything about MC. Is this a viable fix?

Note that I see at least one other negation later in this file that seems like it could trigger the same UB:

return EvaluateSymbolicAdd(Asm, Layout, Addrs, InSet, LHSValue,
  RHSValue.getSymB(), RHSValue.getSymA(),
  -RHSValue.getConstant(), Res);

[spatel]

I've commented out the INT_MIN line in the test to unbreak sanitizer bots:
http://reviews.llvm.org/rL270128

For reference, the intended use of this test is for D20385 .

[majnemer]

lib/MC/MCExpr.cpp

669–673

I'd just go with:

Res = MCValue::get(Value.getSymB(), Value.getSymA(),
                   -(uint64_t)Value.getConstant());

[spatel]

Patch updated:

1. Use David's suggestion of casting to uint64_t.
2. Fix the other similar code where this could happen too.
3. Rebased after rL270128.

[majnemer]

LGTM

lib/MC/MCExpr.cpp

if (!AUE->getSubExpr()->evaluateAsRelocatableImpl(Value, Asm, Layout, Fixup,
       return false;
 
     switch (AUE->getOpcode()) {
     case MCUnaryExpr::LNot:
       if (!Value.isAbsolute())
         return false;
       Res = MCValue::get(!Value.getConstant());
       break;
-    case MCUnaryExpr::Minus:
+    case MCUnaryExpr::Minus: {
       /// -(a - b + const) ==> (b - a - const)
       if (Value.getSymA() && !Value.getSymB())
         return false;
-      Res = MCValue::get(Value.getSymB(), Value.getSymA(),
-                         -Value.getConstant());
+
+      // Avoid undefined behavior: the negation of the minimum constant is the
+      // minimum constant.
+      int64_t ValConst = Value.getConstant();
+      if (ValConst != INT64_MIN)
+        ValConst = -ValConst;
+      
+      Res = MCValue::get(Value.getSymB(), Value.getSymA(), ValConst);
       break;
+    }
     case MCUnaryExpr::Not:
       if (!Value.isAbsolute())
         return false;
       Res = MCValue::get(~Value.getConstant());
       break;
     case MCUnaryExpr::Plus:
       Res = Value;
       break;

test/MC/X86/imm-comments.s

 # RUN: llvm-mc %s -triple=x86_64-unknown-unknown | FileCheck %s
 
 movb  $127, %al
 movb  $-128, %al
 
 movw  $32767, %ax
 movw  $-32768, %ax
 
 movl  $2147483647, %eax
 movl  $-2147483648, %eax
 
 movabsq $9223372036854775807, %rax
-movabsq $-9223372036854775808, %rax
+
+# This line should not induce undefined behavior via negation of INT64_MIN.
+movabsq $-9223372036854775808, %rax 
 
 # CHECK:  movb  $127, %al
 # CHECK:  movb  $-128, %al
 
 # CHECK:  movw  $32767, %ax             # imm = 0x7FFF
 # CHECK:  movw  $-32768, %ax            # imm = 0xFFFFFFFFFFFF8000
 
 # CHECK:  movl  $2147483647, %eax       # imm = 0x7FFFFFFF
 # CHECK:  movl  $-2147483648, %eax      # imm = 0xFFFFFFFF80000000
 
 # CHECK:  movabsq $9223372036854775807, %rax # imm = 0x7FFFFFFFFFFFFFFF
 # CHECK:  movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000