[WebAssembly] Emit br_table for most switch instructions

Summary:
Always convert switches to br_tables unless there is only one case,
which is equivalent to a simple branch. This reduces code size for wasm,
and we defer possible jump table optimizations to the VM.
Addresses PR41502.

Reviewers: kripken, sunfish

Subscribers: dschuff, sbc100, jgravelle-google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60966

llvm-svn: 359038

2 files changed, 34 insertions, 40 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
index 7bad4972c4f..105b1360ac2 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -270,6 +270,11 @@ WebAssemblyTargetLowering::WebAssemblyTargetLowering(
     MaxStoresPerMemset = 1;
     MaxStoresPerMemsetOptSize = 1;
   }
+
+  // Always convert switches to br_tables unless there is only one case, which
+  // is equivalent to a simple branch. This reduces code size for wasm, and we
+  // defer possible jump table optimizations to the VM.
+  setMinimumJumpTableEntries(2);
 }
 
 TargetLowering::AtomicExpansionKind
diff --git a/llvm/test/CodeGen/WebAssembly/cfg-stackify.ll b/llvm/test/CodeGen/WebAssembly/cfg-stackify.ll
index 9f850e8bc72..8c16a465730 100644
--- a/llvm/test/CodeGen/WebAssembly/cfg-stackify.ll
+++ b/llvm/test/CodeGen/WebAssembly/cfg-stackify.ll
@@ -384,18 +384,12 @@ if.end:
 ; CHECK-NEXT: .functype test4 (i32) -> (){{$}}
 ; CHECK:      block   {{$}}
 ; CHECK-NEXT: block   {{$}}
-; CHECK:      br_if       0, $pop{{[0-9]+}}{{$}}
-; CHECK:      br_if       1, $pop{{[0-9]+}}{{$}}
-; CHECK:      br          1{{$}}
-; CHECK-NEXT: .LBB{{[0-9]+}}_3:
-; CHECK-NEXT: end_block{{$}}
-; CHECK-NEXT: block   {{$}}
-; CHECK:      br_if 0, $pop{{[0-9]+}}{{$}}
-; CHECK:      br_if 1, $pop{{[0-9]+}}{{$}}
-; CHECK-NEXT: .LBB{{[0-9]+}}_5:
+; CHECK:      br_if     0, $pop{{[0-9]+}}{{$}}
+; CHECK:      br        1{{$}}
+; CHECK-NEXT: .LBB{{[0-9]+}}_2:
 ; CHECK-NEXT: end_block{{$}}
-; CHECK-NEXT: return{{$}}
-; CHECK-NEXT: .LBB{{[0-9]+}}_6:
+; CHECK-NEXT: br_table   $0, 0, 0, 0, 0, 0, 0{{$}}
+; CHECK-NEXT: .LBB{{[0-9]+}}_3:
 ; CHECK-NEXT: end_block{{$}}
 ; CHECK-NEXT: return{{$}}
 define void @test4(i32 %t) {
@@ -646,29 +640,30 @@ end:
 ; CHECK-LABEL: test10:
 ; CHECK:       .LBB{{[0-9]+}}_1:
 ; CHECK-NEXT:  loop    {{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if    0, {{[^,]+}}{{$}}
+; CHECK:       br_if     0, {{[^,]+}}{{$}}
 ; CHECK:       .LBB{{[0-9]+}}_3:
 ; CHECK-NEXT:  block   {{$}}
 ; CHECK-NEXT:  loop    {{$}}
-; CHECK-NOT:   block
 ; CHECK:       .LBB{{[0-9]+}}_4:
 ; CHECK-NEXT:  loop    {{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if    0, {{[^,]+}}{{$}}
+; CHECK:       br_if     0, {{[^,]+}}{{$}}
 ; CHECK-NEXT:  end_loop{{$}}
-; CHECK:       br_if    1, {{[^,]+}}{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if    0, {{[^,]+}}{{$}}
+; CHECK-NEXT:  block   {{$}}
+; CHECK:       br_if     0, {{[^,]+}}{{$}}
+; CHECK:       br        3{{$}}
+; CHECK-NEXT:  .LBB{{[0-9]+}}_7:
+; CHECK-NEXT:  end_block{{$}}
+; CHECK:       block   {{$}}
+; CHECK-NEXT:  br_table   $0, 0, 3, 1, 2, 0
+; CHECK-NEXT:  .LBB{{[0-9]+}}_8:
+; CHECK-NEXT:  end_block{{$}}
 ; CHECK-NEXT:  end_loop{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if    1, {{[^,]+}}{{$}}
 ; CHECK-NEXT:  return{{$}}
 ; CHECK-NEXT:  .LBB{{[0-9]+}}_9:
 ; CHECK-NEXT:  end_block{{$}}
-; CHECK-NOT:   block
-; CHECK:       br       0{{$}}
+; CHECK:       br        0{{$}}
 ; CHECK-NEXT:  .LBB{{[0-9]+}}_10:
+; CHECK-NEXT:  end_loop{{$}}
 define void @test10() {
 bb0:
   br label %bb1
@@ -772,31 +767,25 @@ bb8:
 
 ; CHECK-LABEL: test12:
 ; CHECK:       .LBB{{[0-9]+}}_1:
-; CHECK-NEXT:  block   {{$}}
 ; CHECK-NEXT:  loop    {{$}}
-; CHECK-NOT:   block
-; CHECK:       block   {{$}}
 ; CHECK-NEXT:  block   {{$}}
-; CHECK:       br_if       0, {{[^,]+}}{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if       1, {{[^,]+}}{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if       1, {{[^,]+}}{{$}}
-; CHECK-NEXT:  br          3{{$}}
+; CHECK-NEXT:  block   {{$}}
+; CHECK-NEXT:  block   {{$}}
+; CHECK:       br_if     0, {{[^,]+}}{{$}}
+; CHECK:       br_if     2, {{[^,]+}}{{$}}
+; CHECK:       br_if     1, {{[^,]+}}{{$}}
+; CHECK-NEXT:  br        2{{$}}
 ; CHECK-NEXT:  .LBB{{[0-9]+}}_4:
 ; CHECK-NEXT:  end_block{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if       0, {{[^,]+}}{{$}}
-; CHECK-NOT:   block
-; CHECK:       br_if       2, {{[^,]+}}{{$}}
+; CHECK-NEXT:  br_table   $2, 1, 0, 0, 0, 1, 1{{$}}
+; CHECK-NEXT:  .LBB{{[0-9]+}}_5:
+; CHECK-NEXT:  end_block{{$}}
+; CHECK-NEXT:  return{{$}}
 ; CHECK-NEXT:  .LBB{{[0-9]+}}_6:
 ; CHECK-NEXT:  end_block{{$}}
-; CHECK-NOT:   block
-; CHECK:       br          0{{$}}
+; CHECK:       br        0{{$}}
 ; CHECK-NEXT:  .LBB{{[0-9]+}}_7:
 ; CHECK-NEXT:  end_loop{{$}}
-; CHECK-NEXT:  end_block{{$}}
-; CHECK-NEXT:  return{{$}}
 define void @test12(i8* %arg) {
 bb:
   br label %bb1