[llvm] c5d0123 - [WebAssembly] Make BR_TABLE non-duplicable
Thomas Lively via llvm-commits
llvm-commits at lists.llvm.org
Thu Jun 11 15:11:53 PDT 2020
Author: Thomas Lively
Date: 2020-06-11T15:11:45-07:00
New Revision: c5d012341e58bf5e4300024a7e4f4e509c08463f
URL: https://github.com/llvm/llvm-project/commit/c5d012341e58bf5e4300024a7e4f4e509c08463f
DIFF: https://github.com/llvm/llvm-project/commit/c5d012341e58bf5e4300024a7e4f4e509c08463f.diff
LOG: [WebAssembly] Make BR_TABLE non-duplicable
Summary:
After their range checks were removed in 7f50c15be5c0, br_tables
started being duplicated into their predecessors by tail
folding. Unfortunately, when the br_tables were in loops this
transformation introduced bad irreducible control flow which was later
expanded into even more br_tables. This commit abuses the
`isNotDuplicable` property to prevent this irreducible control flow
from being introduced. This change saves a few dozen bytes of code
size and has a negligible affect on performance for most of the large
Emscripten benchmarks, but can improve performance significantly on
microbenchmarks of switches in loops.
Reviewers: aheejin, dschuff
Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D81628
Added:
llvm/test/CodeGen/WebAssembly/switch-in-loop.ll
Modified:
llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
llvm/test/CodeGen/WebAssembly/indirectbr.ll
Removed:
################################################################################
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
index 03b8004315a0..171dd9a67beb 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
@@ -40,21 +40,25 @@ def brlist : Operand<i32> {
let PrintMethod = "printBrList";
}
-// TODO: SelectionDAG's lowering insists on using a pointer as the index for
-// jump tables, so in practice we don't ever use BR_TABLE_I64 in wasm32 mode
-// currently.
-let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
+// Duplicating a BR_TABLE is almost never a good idea. In particular, it can
+// lead to some nasty irreducibility due to tail merging when the br_table is in
+// a loop.
+let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1, isNotDuplicable = 1 in {
+
defm BR_TABLE_I32 : I<(outs), (ins I32:$index, variable_ops),
(outs), (ins brlist:$brl),
[(WebAssemblybr_table I32:$index)],
"br_table \t$index", "br_table \t$brl",
0x0e>;
+// TODO: SelectionDAG's lowering insists on using a pointer as the index for
+// jump tables, so in practice we don't ever use BR_TABLE_I64 in wasm32 mode
+// currently.
defm BR_TABLE_I64 : I<(outs), (ins I64:$index, variable_ops),
(outs), (ins brlist:$brl),
[(WebAssemblybr_table I64:$index)],
"br_table \t$index", "br_table \t$brl",
0x0e>;
-} // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
+} // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1, isNotDuplicable = 1
// This is technically a control-flow instruction, since all it affects is the
// IP.
diff --git a/llvm/test/CodeGen/WebAssembly/indirectbr.ll b/llvm/test/CodeGen/WebAssembly/indirectbr.ll
index da737613f30f..2cf41e4c5880 100644
--- a/llvm/test/CodeGen/WebAssembly/indirectbr.ll
+++ b/llvm/test/CodeGen/WebAssembly/indirectbr.ll
@@ -13,36 +13,21 @@ target triple = "wasm32"
; Just check the barest skeleton of the structure
; CHECK-LABEL: test1:
-; CHECK: block
-; CHECK: block
-; CHECK: block
-; CHECK: block
; CHECK: i32.load
; CHECK: i32.load
-; CHECK: i32.const
-; CHECK: i32.add $push[[DEST:.+]]=
-; CHECK: br_table $pop[[DEST]]
-; CHECK: end_block
-; CHECK: end_block
-; CHECK: end_block
-; CHECK: end_block
; CHECK: loop
; CHECK: block
; CHECK: block
; CHECK: block
; CHECK: block
-; CHECK: br_table ${{[^,]+}}, 0, 1, 2, 2
-; CHECK: end_block
-; CHECK: end_block
-; CHECK: end_block
-; CHECK: block
-; CHECK: block
-; CHECK: block
; CHECK: br_table ${{[^,]+}}, 1, 2, 0
; CHECK: end_block
; CHECK: end_block
; CHECK: end_block
+; CHECK: end_block
+; CHECK: br
; CHECK: end_loop
+; CHECK: end_function
; CHECK: test1.targets:
; CHECK-NEXT: .int32
; CHECK-NEXT: .int32
diff --git a/llvm/test/CodeGen/WebAssembly/switch-in-loop.ll b/llvm/test/CodeGen/WebAssembly/switch-in-loop.ll
new file mode 100644
index 000000000000..c466f291f475
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/switch-in-loop.ll
@@ -0,0 +1,77 @@
+; RUN: llc < %s -asm-verbose=false -verify-machineinstrs | FileCheck %s
+
+;; Test that a small but nontrivial switch in a loop (like in a
+;; bytecode interpreter) lowers reasonably without any irreducible
+;; control flow being introduced.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32"
+
+declare void @a(i32*)
+declare void @b(i32*)
+
+; CHECK-LABEL: switch_in_loop:
+; CHECK-NEXT: .functype switch_in_loop (i32, i32) -> (i32)
+; CHECK: global.get __stack_pointer
+; CHECK: global.set __stack_pointer
+; CHECK: block
+; CHECK: br_if 0
+; CHECK: .LBB0_2:
+; CHECK: loop
+; CHECK: block
+; CHECK: block
+; CHECK: block
+; CHECK: br_table {0, 1, 2}
+; CHECK: .LBB0_3:
+; CHECK: end_block
+; CHECK: call a
+; CHECK: br 1
+; CHECK: .LBB0_4:
+; CHECK: end_block
+; CHECK: call b
+; CHECK: .LBB0_5:
+; CHECK: end_block
+; CHECK: br_if 0
+; CHECK: end_loop
+; CHECK: .LBB0_7:
+; CHECK: end_block
+; CHECK: global.set __stack_pointer
+; CHECK: end_function
+define i32 @switch_in_loop(i32* %ops, i32 %len) {
+entry:
+ %res = alloca i32
+ %0 = bitcast i32* %res to i8*
+ store i32 0, i32* %res
+ %cmp6 = icmp sgt i32 %len, 0
+ br i1 %cmp6, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup.loopexit: ; preds = %sw.epilog
+ %.pre = load i32, i32* %res
+ br label %for.cond.cleanup
+
+for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry
+ %1 = phi i32 [ %.pre, %for.cond.cleanup.loopexit ], [ 0, %entry ]
+ ret i32 %1
+
+for.body: ; preds = %entry, %sw.epilog
+ %i.07 = phi i32 [ %inc, %sw.epilog ], [ 0, %entry ]
+ %arrayidx = getelementptr inbounds i32, i32* %ops, i32 %i.07
+ %2 = load i32, i32* %arrayidx
+ switch i32 %2, label %sw.epilog [
+ i32 0, label %sw.bb
+ i32 1, label %sw.bb1
+ ]
+
+sw.bb: ; preds = %for.body
+ call void @a(i32* nonnull %res)
+ br label %sw.epilog
+
+sw.bb1: ; preds = %for.body
+ call void @b(i32* nonnull %res)
+ br label %sw.epilog
+
+sw.epilog: ; preds = %for.body, %sw.bb1, %sw.bb
+ %inc = add nuw nsw i32 %i.07, 1
+ %exitcond = icmp eq i32 %inc, %len
+ br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body
+}
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