[MC] Don't crash on division by zero

[davide]

Summary

Maybe there's a better way to handle this case (in the Parser?), but at least we now don't crash with FPE on such cases. Also, FWIW, gas seems to handle this in the same exact way (+ emitting a warning)

[• rafael]

+ Check that llvm-mc doesn't crash on division by zero.
+ RUN: llvm-mc -triple x86_64-pc-linux-gnu %s -o - 2>&1
+
+.int 1/0

Does it work with -filetype=obj? What is output?

Index: lib/MC/MCExpr.cpp

• lib/MC/MCExpr.cpp

+++ lib/MC/MCExpr.cpp
@@ -734,6 +734,11 @@

// width, and gas defines the result of comparisons differently from
// Apple as.
int64_t LHS = LHSValue.getConstant(), RHS = RHSValue.getConstant();

+
+ // Handle division by zero.
+ if (ABE->getOpcode() == MCBinaryExpr::Div && RHS == 0)
+ return false;
+

Move the check to the "case MCBinaryExpr::Div:" bellow.

int64_t Result = 0;
switch (ABE->getOpcode()) {
case MCBinaryExpr::AShr: Result = LHS >> RHS; break;

Cheers,
Rafael

lib/MC/MCExpr.cpp

                                    RHSValue.getConstant(), Res);
       }
     }
 
     // FIXME: We need target hooks for the evaluation. It may be limited in
     // width, and gas defines the result of comparisons differently from
     // Apple as.
     int64_t LHS = LHSValue.getConstant(), RHS = RHSValue.getConstant();
+
+    // Handle division by zero.
+    if (ABE->getOpcode() == MCBinaryExpr::Div && RHS == 0)
+      return false;
+
     int64_t Result = 0;
     switch (ABE->getOpcode()) {
     case MCBinaryExpr::AShr: Result = LHS >> RHS; break;
     case MCBinaryExpr::Add:  Result = LHS + RHS; break;
     case MCBinaryExpr::And:  Result = LHS & RHS; break;
     case MCBinaryExpr::Div:  Result = LHS / RHS; break;
     case MCBinaryExpr::EQ:   Result = LHS == RHS; break;
     case MCBinaryExpr::GT:   Result = LHS > RHS; break;

test/MC/ELF/div-by-zero.s

+// Check that llvm-mc doesn't crash on division by zero.
+// RUN: llvm-mc -triple x86_64-pc-linux-gnu %s -o - 2>&1 
+
+.int 1/0