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[X86] Improve code size on X86 segment moves 

Moves of a value to a segment register from a 16-bit register is
equivalent to one from it's corresponding 32-bit register. Match gas's
behavior and rewrite instructions to the shorter of equivalent forms.

Reviewers: rnk, ab

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23166

llvm-svn: 278031
This commit is contained in:
Nirav Dave 2016-08-08 18:01:04 +00:00
parent 9d09275fa6
commit f45fd2ba87
3 changed files with 50 additions and 2 deletions
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24
llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
View File

@ -2331,6 +2331,30 @@ bool X86AsmParser::ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
static_cast<X86Operand &>(*Operands[0]).setTokenValue(Repl);
}
// Moving a 32 or 16 bit value into a segment register has the same
// behavior. Modify such instructions to always take shorter form.
if ((Name == "mov" || Name == "movw" || Name == "movl") &&
(Operands.size() == 3)) {
X86Operand &Op1 = (X86Operand &)*Operands[1];
X86Operand &Op2 = (X86Operand &)*Operands[2];
SMLoc Loc = Op1.getEndLoc();
if (Op1.isReg() && Op2.isReg() &&
X86MCRegisterClasses[X86::SEGMENT_REGRegClassID].contains(
Op2.getReg()) &&
(X86MCRegisterClasses[X86::GR16RegClassID].contains(Op1.getReg()) ||
X86MCRegisterClasses[X86::GR32RegClassID].contains(Op1.getReg()))) {
// Change instruction name to match new instruction.
if (Name != "mov" && Name[3] == (is16BitMode() ? 'l' : 'w')) {
Name = is16BitMode() ? "movw" : "movl";
Operands[0] = X86Operand::CreateToken(Name, NameLoc);
}
// Select the correct equivalent 16-/32-bit source register.
unsigned Reg =
getX86SubSuperRegisterOrZero(Op1.getReg(), is16BitMode() ? 16 : 32);
Operands[1] = X86Operand::CreateReg(Reg, Loc, Loc);
}
}
// This is a terrible hack to handle "out[s]?[bwl]? %al, (%dx)" ->
// "outb %al, %dx". Out doesn't take a memory form, but this is a widely
// documented form in various unofficial manuals, so a lot of code uses it.

16
llvm/test/MC/X86/x86-16.s
View File

@ -256,10 +256,22 @@ cmovnae %bx,%bx
// CHECK: encoding: [0x67,0x8c,0x08]
movw %cs, (%eax)
// CHECK: movl %eax, %cs
// CHECK: encoding: [0x66,0x8e,0xc8]
// CHECK: movw %ax, %cs
// CHECK: encoding: [0x8e,0xc8]
movl %eax, %cs
// CHECK: movw %ax, %cs
// CHECK: encoding: [0x8e,0xc8]
mov %eax, %cs
// CHECK: movw %ax, %cs
// CHECK: encoding: [0x8e,0xc8]
movw %ax, %cs
// CHECK: movw %ax, %cs
// CHECK: encoding: [0x8e,0xc8]
mov %ax, %cs
// CHECK: movl (%eax), %cs
// CHECK: encoding: [0x67,0x66,0x8e,0x08]
movl (%eax), %cs

12
llvm/test/MC/X86/x86-32.s
View File

@ -367,6 +367,18 @@ cmovnae %bx,%bx
// CHECK: encoding: [0x8e,0xc8]
movl %eax, %cs
// CHECK: movl %eax, %cs
// CHECK: encoding: [0x8e,0xc8]
movw %ax, %cs
// CHECK: movl %eax, %cs
// CHECK: encoding: [0x8e,0xc8]
mov %eax, %cs
// CHECK: movl %eax, %cs
// CHECK: encoding: [0x8e,0xc8]
mov %ax, %cs
// CHECK: movl (%eax), %cs
// CHECK: encoding: [0x8e,0x08]
movl (%eax), %cs