[PATCH] D20434: [MCExpr] avoid UB via negation of INT_MIN

[Sanjay Patel via llvm-commits]

spatel updated this revision to Diff 57864.
spatel added a comment.

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 http://reviews.llvm.org/rL270128.


http://reviews.llvm.org/D20434

Files:
  lib/MC/MCExpr.cpp
  test/MC/X86/imm-comments.s

Index: test/MC/X86/imm-comments.s
===================================================================
--- test/MC/X86/imm-comments.s
+++ test/MC/X86/imm-comments.s
@@ -11,8 +11,8 @@
 
 movabsq	$9223372036854775807, %rax
 
-# FIXME: This line causes UB failure.
-# 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
@@ -24,5 +24,5 @@
 # CHECK:  movl  $-2147483648, %eax      # imm = 0xFFFFFFFF80000000
 
 # CHECK:  movabsq $9223372036854775807, %rax # imm = 0x7FFFFFFFFFFFFFFF
-# FIXME-CHECK:  movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000
+# CHECK:  movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000
 
Index: lib/MC/MCExpr.cpp
===================================================================
--- lib/MC/MCExpr.cpp
+++ lib/MC/MCExpr.cpp
@@ -663,8 +663,10 @@
       /// -(a - b + const) ==> (b - a - const)
       if (Value.getSymA() && !Value.getSymB())
         return false;
+
+      // The cast avoids undefined behavior if the constant is INT64_MIN.
       Res = MCValue::get(Value.getSymB(), Value.getSymA(),
-                         -Value.getConstant());
+                         -(uint64_t)Value.getConstant());
       break;
     case MCUnaryExpr::Not:
       if (!Value.isAbsolute())
@@ -697,9 +699,10 @@
         return false;
       case MCBinaryExpr::Sub:
         // Negate RHS and add.
+        // The cast avoids undefined behavior if the constant is INT64_MIN.
         return EvaluateSymbolicAdd(Asm, Layout, Addrs, InSet, LHSValue,
                                    RHSValue.getSymB(), RHSValue.getSymA(),
-                                   -RHSValue.getConstant(), Res);
+                                   -(uint64_t)RHSValue.getConstant(), Res);
 
       case MCBinaryExpr::Add:
         return EvaluateSymbolicAdd(Asm, Layout, Addrs, InSet, LHSValue,


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