lib/Target/X86/InstSelectSimple.cpp: Start counting arguments with 2,

because arguments start two stack slots off of EBP. Break out of the
 for loop once the argument is found. Increment the counter at the end
 of the loop instead of the beginning. Use addRegOffset and compute
 the scale * index part at compile time instead of using the fancy
 load instruction. Just because an instruction set has wacky addressing
 modes doesn't mean we ought to use them (at least, if you believe Dave
 Patterson).

lib/Target/X86/X86InstrBuilder.h: Add some comments.

test/Regression/Jello/test-loadstore.ll:  Let main return int 0.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4999 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/InstSelectSimple.cpp

@@ -159,23 +159,24 @@ namespace {
       } else if (Argument *A = dyn_cast<Argument>(V)) {
 	// Find the position of the argument in the argument list.
 	const Function *f = F->getFunction ();
-	int counter = 0, argPosition = -1;
+	// The function's arguments look like this:
+	// [EBP]     -- copy of old EBP
+	// [EBP + 4] -- return address
+	// [EBP + 8] -- first argument (leftmost lexically)
+	// So we want to start with counter = 2.
+	int counter = 2, argPosition = -1;
 	for (Function::const_aiterator ai = f->abegin (), ae = f->aend ();
 	     ai != ae; ++ai) {
-	  ++counter;
 	  if (&(*ai) == A) {
 	    argPosition = counter;
+	    break; // Only need to find it once. ;-)
 	  }
+	  ++counter;
 	}
 	assert (argPosition != -1
 		&& "Argument not found in current function's argument list");
 	// Load it out of the stack frame at EBP + 4*argPosition.
-	// (First, load Reg with argPosition, then load Reg with DWORD
-	// PTR [EBP + 4*Reg].)
-	BuildMI (BB, X86::MOVir32, 1, Reg).addZImm (argPosition);
-	BuildMI (BB, X86::MOVmr32, 4,
-		 Reg).addReg (X86::EBP).addZImm (4).addReg (Reg).addSImm (0);
-        // std::cerr << "ERROR: Arguments not implemented in SimpleInstSel\n";
+	addRegOffset (BuildMI (BB, X86::MOVmr32, 4, Reg), X86::EBP, 4*argPosition);
       }
 
       return Reg;

lib/Target/X86/X86ISelSimple.cpp

@@ -159,23 +159,24 @@ namespace {
       } else if (Argument *A = dyn_cast<Argument>(V)) {
 	// Find the position of the argument in the argument list.
 	const Function *f = F->getFunction ();
-	int counter = 0, argPosition = -1;
+	// The function's arguments look like this:
+	// [EBP]     -- copy of old EBP
+	// [EBP + 4] -- return address
+	// [EBP + 8] -- first argument (leftmost lexically)
+	// So we want to start with counter = 2.
+	int counter = 2, argPosition = -1;
 	for (Function::const_aiterator ai = f->abegin (), ae = f->aend ();
 	     ai != ae; ++ai) {
-	  ++counter;
 	  if (&(*ai) == A) {
 	    argPosition = counter;
+	    break; // Only need to find it once. ;-)
 	  }
+	  ++counter;
 	}
 	assert (argPosition != -1
 		&& "Argument not found in current function's argument list");
 	// Load it out of the stack frame at EBP + 4*argPosition.
-	// (First, load Reg with argPosition, then load Reg with DWORD
-	// PTR [EBP + 4*Reg].)
-	BuildMI (BB, X86::MOVir32, 1, Reg).addZImm (argPosition);
-	BuildMI (BB, X86::MOVmr32, 4,
-		 Reg).addReg (X86::EBP).addZImm (4).addReg (Reg).addSImm (0);
-        // std::cerr << "ERROR: Arguments not implemented in SimpleInstSel\n";
+	addRegOffset (BuildMI (BB, X86::MOVmr32, 4, Reg), X86::EBP, 4*argPosition);
       }
 
       return Reg;

lib/Target/X86/X86InstrBuilder.h

@@ -9,6 +9,9 @@
 // up behind an easier to use interface makes sense.  Descriptions of the
 // functions are included below.
 //
+// For reference, the order of operands for memory references is:
+// (Operand), Base, Scale, Index, Displacement.
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef X86INSTRBUILDER_H
@@ -17,18 +20,19 @@
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 
 /// addDirectMem - This function is used to add a direct memory reference to the
-/// current instruction.  Because memory references are always represented with
-/// four values, this adds: Reg, [1, NoReg, 0] to the instruction
-///
+/// current instruction -- that is, a dereference of an address in a register, with
+/// no scale, index or displacement. An example is: DWORD PTR [EAX].
 inline const MachineInstrBuilder &addDirectMem(const MachineInstrBuilder &MIB,
                                                unsigned Reg) {
+  // Because memory references are always represented with four
+  // values, this adds: Reg, [1, NoReg, 0] to the instruction.
   return MIB.addReg(Reg).addZImm(1).addMReg(0).addSImm(0);
 }
 
 
-/// addRegOffset -
-/// 
-///
+/// addRegOffset - This function is used to add a memory reference of
+/// the form [Reg + Offset], i.e., one with no scale or index, but
+/// with a displacement. An example is: DWORD PTR [EAX + 4].
 inline const MachineInstrBuilder &addRegOffset(const MachineInstrBuilder &MIB,
                                                unsigned Reg, unsigned Offset) {
   return MIB.addReg(Reg).addZImm(1).addMReg(0).addSImm(Offset);

test/ExecutionEngine/test-loadstore.ll

@@ -11,10 +11,10 @@ void %test(sbyte* %P, short* %P, int* %P) {
 	ret void
 }
 
-void %main() {
+int %main() {
 	%A = alloca sbyte
 	%B = alloca short
 	%C = alloca int
 	call void %test(sbyte* %A, short* %B, int* %C)
-	ret void
+	ret int 0
 }