[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

llvm/trunk/lib/MC/MCExpr.cpp

case Binary: {
     // width, and gas defines the result of comparisons differently from
     // Apple as.
     int64_t LHS = LHSValue.getConstant(), RHS = RHSValue.getConstant();
     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::Div: {
+      // Handle division by zero. gas just emits a warning and keeps going,
+      // we try to be stricter.
+      // FIXME: Currently the caller of this function has no way to understand
+      // we're bailing out because of 'division by zero'. Therefore, it will
+      // emit a 'expected relocatable expression' error. It would be nice to
+      // change this code to emit a better diagnostic.
+      if (RHS == 0)
+        return false;
+      Result = LHS / RHS; break;
+    }
     case MCBinaryExpr::EQ:   Result = LHS == RHS; break;
     case MCBinaryExpr::GT:   Result = LHS > RHS; break;
     case MCBinaryExpr::GTE:  Result = LHS >= RHS; break;
     case MCBinaryExpr::LAnd: Result = LHS && RHS; break;
     case MCBinaryExpr::LOr:  Result = LHS || RHS; break;
     case MCBinaryExpr::LShr: Result = uint64_t(LHS) >> uint64_t(RHS); break;
     case MCBinaryExpr::LT:   Result = LHS < RHS; break;
     case MCBinaryExpr::LTE:  Result = LHS <= RHS; break;

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

+// Check that llvm-mc doesn't crash on division by zero.
+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s 2> %t
+// RUN: FileCheck -input-file %t %s
+
+// CHECK: expected relocatable expression
+.int 1/0