[AsmPrinter] Further restrict expressions supported in global initial…

…izers

lowerConstant() currently accepts a number of constant expressions
which have corresponding MC expressions, but which cannot be
evaluated as a relocatable expression (unless the operands are
constant, in which case we'll just fold the expression to a constant).

The motivation here is to clarify which constant expressions are
really needed for https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179,
and in particular clarify that we do not need to support any
division expressions, which are particularly problematic.

Differential Revision: https://reviews.llvm.org/D127972

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -2722,6 +2722,9 @@ const MCExpr *AsmPrinter::lowerConstant(const Constant *CV) {
     llvm_unreachable("Unknown constant value to lower!");
   }
 
+  // The constant expression opcodes are limited to those that are necessary
+  // to represent relocations on supported targets. Expressions involving only
+  // constant addresses are constant folded instead.
   switch (CE->getOpcode()) {
   case Instruction::AddrSpaceCast: {
     const Constant *Op = CE->getOperand(0);
@@ -2839,34 +2842,17 @@ const MCExpr *AsmPrinter::lowerConstant(const Constant *CV) {
         return RelocExpr;
       }
     }
+
+    const MCExpr *LHS = lowerConstant(CE->getOperand(0));
+    const MCExpr *RHS = lowerConstant(CE->getOperand(1));
+    return MCBinaryExpr::createSub(LHS, RHS, Ctx);
+    break;
   }
-  // else fallthrough
-  LLVM_FALLTHROUGH;
-
-  // The MC library also has a right-shift operator, but it isn't consistently
-  // signed or unsigned between different targets.
-  case Instruction::Add:
-  case Instruction::Mul:
-  case Instruction::SDiv:
-  case Instruction::SRem:
-  case Instruction::Shl:
-  case Instruction::And:
-  case Instruction::Or:
-  case Instruction::Xor: {
+
+  case Instruction::Add: {
     const MCExpr *LHS = lowerConstant(CE->getOperand(0));
     const MCExpr *RHS = lowerConstant(CE->getOperand(1));
-    switch (CE->getOpcode()) {
-    default: llvm_unreachable("Unknown binary operator constant cast expr");
-    case Instruction::Add: return MCBinaryExpr::createAdd(LHS, RHS, Ctx);
-    case Instruction::Sub: return MCBinaryExpr::createSub(LHS, RHS, Ctx);
-    case Instruction::Mul: return MCBinaryExpr::createMul(LHS, RHS, Ctx);
-    case Instruction::SDiv: return MCBinaryExpr::createDiv(LHS, RHS, Ctx);
-    case Instruction::SRem: return MCBinaryExpr::createMod(LHS, RHS, Ctx);
-    case Instruction::Shl: return MCBinaryExpr::createShl(LHS, RHS, Ctx);
-    case Instruction::And: return MCBinaryExpr::createAnd(LHS, RHS, Ctx);
-    case Instruction::Or:  return MCBinaryExpr::createOr (LHS, RHS, Ctx);
-    case Instruction::Xor: return MCBinaryExpr::createXor(LHS, RHS, Ctx);
-    }
+    return MCBinaryExpr::createAdd(LHS, RHS, Ctx);
   }
   }
 }

llvm/test/CodeGen/X86/nonconst-static-div.ll

@@ -0,0 +1,12 @@
+; RUN: not --crash llc -mtriple=i686-linux-gnu < %s 2>&1 | FileCheck %s
+
+; Targets only support a limited set of relocations. Make sure that unusual
+; constant expressions (and in particular potentially trapping ones involving
+; division) are already rejected when lowering the Constant to the MC layer,
+; rather than only when trying to emit a relocation. This makes sure that an
+; error is reported when targeting assembly (without -filetype=obj).
+
+@g = external global i32
+@g2 = global i64 sdiv (i64 3, i64 ptrtoint (ptr @g to i64))
+
+; CHECK: Unsupported expression in static initializer: sdiv

llvm/test/CodeGen/X86/ptrtoint-constexpr.ll

@@ -9,6 +9,6 @@
 
 ; CHECK:	.globl x
 ; CHECK: x:
-; CHECK: .quad	((0+1)&4294967295)*3
+; CHECK: .quad	3
 
 @x = global i64 mul (i64 3, i64 ptrtoint (ptr getelementptr (i2, ptr null, i64 1) to i64))