X86: Shrink certain forms of movsx.

In particular:
movsbw %al, %ax   --> cbtw
movswl %ax, %eax  --> cwtl
movslq %eax, %rax --> cltq

According to Intel's manual those have the same performance characteristics but
come with a smaller encoding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186174 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 62 insertions, 6 deletions

diff --git a/lib/Target/X86/X86MCInstLower.cpp b/lib/Target/X86/X86MCInstLower.cpp
index 3fea4cb560..c7c00b536c 100644
--- a/lib/Target/X86/X86MCInstLower.cpp
+++ b/lib/Target/X86/X86MCInstLower.cpp
@@ -254,6 +254,34 @@ static void SimplifyShortImmForm(MCInst &Inst, unsigned Opcode) {
   Inst.addOperand(Saved);
 }
 
+/// \brief If a movsx instruction has a shorter encoding for the used register
+/// simplify the instruction to use it instead.
+static void SimplifyMOVSX(MCInst &Inst) {
+  unsigned NewOpcode = 0;
+  unsigned Op0 = Inst.getOperand(0).getReg(), Op1 = Inst.getOperand(1).getReg();
+  switch (Inst.getOpcode()) {
+  default:
+    llvm_unreachable("Unexpected instruction!");
+  case X86::MOVSX16rr8:  // movsbw %al, %ax   --> cbtw
+    if (Op0 == X86::AX && Op1 == X86::AL)
+      NewOpcode = X86::CBW;
+    break;
+  case X86::MOVSX32rr16: // movswl %ax, %eax  --> cwtl
+    if (Op0 == X86::EAX && Op1 == X86::AX)
+      NewOpcode = X86::CWDE;
+    break;
+  case X86::MOVSX64rr32: // movslq %eax, %rax --> cltq
+    if (Op0 == X86::RAX && Op1 == X86::EAX)
+      NewOpcode = X86::CDQE;
+    break;
+  }
+
+  if (NewOpcode != 0) {
+    Inst = MCInst();
+    Inst.setOpcode(NewOpcode);
+  }
+}
+
 /// \brief Simplify things like MOV32rm to MOV32o32a.
 static void SimplifyShortMoveForm(X86AsmPrinter &Printer, MCInst &Inst,
                                   unsigned Opcode) {
@@ -557,6 +585,13 @@ ReSimplify:
   case X86::XOR32ri:    SimplifyShortImmForm(OutMI, X86::XOR32i32);  break;
   case X86::XOR64ri32:  SimplifyShortImmForm(OutMI, X86::XOR64i32);  break;
 
+  // Try to shrink some forms of movsx.
+  case X86::MOVSX16rr8:
+  case X86::MOVSX32rr16:
+  case X86::MOVSX64rr32:
+    SimplifyMOVSX(OutMI);
+    break;
+
   case X86::MORESTACK_RET:
     OutMI.setOpcode(X86::RET);
     break;
diff --git a/test/CodeGen/X86/fast-isel-ret-ext.ll b/test/CodeGen/X86/fast-isel-ret-ext.ll
index fd768cb9b3..395ad1e616 100644
--- a/test/CodeGen/X86/fast-isel-ret-ext.ll
+++ b/test/CodeGen/X86/fast-isel-ret-ext.ll
@@ -26,7 +26,7 @@ define signext i16 @test4(i32 %y) nounwind {
   %conv = trunc i32 %y to i16
   ret i16 %conv
   ; CHECK: test4:
-  ; CHECK: movswl {{.*}}, %eax
+  ; CHECK: {{(movswl.%.x, %eax|cwtl)}}
 }
 
 define zeroext i1 @test5(i32 %y) nounwind {
diff --git a/test/CodeGen/X86/mcinst-lowering.ll b/test/CodeGen/X86/mcinst-lowering.ll
index 1ef5a971ba..391f9398f0 100644
--- a/test/CodeGen/X86/mcinst-lowering.ll
+++ b/test/CodeGen/X86/mcinst-lowering.ll
@@ -24,3 +24,21 @@ if.end:                                           ; preds = %entry
 return:                                           ; preds = %entry
   ret i32 0
 }
+
+define i32 @f1() nounwind {
+  %ax = tail call i16 asm sideeffect "", "={ax},~{dirflag},~{fpsr},~{flags}"()
+  %conv = sext i16 %ax to i32
+  ret i32 %conv
+
+; CHECK: f1:
+; CHECK: cwtl ## encoding: [0x98]
+}
+
+define i64 @f2() nounwind {
+  %eax = tail call i32 asm sideeffect "", "={ax},~{dirflag},~{fpsr},~{flags}"()
+  %conv = sext i32 %eax to i64
+  ret i64 %conv
+
+; CHECK: f2:
+; CHECK: cltq ## encoding: [0x48,0x98]
+}
diff --git a/test/CodeGen/X86/shl_elim.ll b/test/CodeGen/X86/shl_elim.ll
index 83e1eb5c39..e99ecac872 100644
--- a/test/CodeGen/X86/shl_elim.ll
+++ b/test/CodeGen/X86/shl_elim.ll
@@ -1,6 +1,4 @@
-; RUN: llc < %s -march=x86 | grep "movl	8(.esp), %eax"
-; RUN: llc < %s -march=x86 | grep "shrl	.eax"
-; RUN: llc < %s -march=x86 | grep "movswl	.ax, .eax"
+; RUN: llc < %s -march=x86 | FileCheck %s
 
 define i32 @test1(i64 %a) nounwind {
         %tmp29 = lshr i64 %a, 24                ; <i64> [#uses=1]
@@ -9,5 +7,10 @@ define i32 @test1(i64 %a) nounwind {
         %tmp45 = trunc i32 %tmp410 to i16               ; <i16> [#uses=1]
         %tmp456 = sext i16 %tmp45 to i32                ; <i32> [#uses=1]
         ret i32 %tmp456
+
+; CHECK: test1:
+; CHECK: movl 8(%esp), %eax
+; CHECK: shrl %eax
+; CHECK: cwtl
 }
 
diff --git a/test/CodeGen/X86/widen_conv-2.ll b/test/CodeGen/X86/widen_conv-2.ll
index 969cb512be..db8fa93dc7 100644
--- a/test/CodeGen/X86/widen_conv-2.ll
+++ b/test/CodeGen/X86/widen_conv-2.ll
@@ -1,6 +1,6 @@
 ; RUN: llc < %s -march=x86 -mattr=+sse42 | FileCheck %s
-; CHECK: movswl
-; CHECK: movswl
+; CHECK: cwtl
+; CHECK: cwtl
 
 ; sign extension v2i32 to v2i16