[PATCH] D136237: [BasicBlockSections] avoid insertting redundant branch to fall through blocks
Sinan Lin via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Wed Nov 9 19:32:18 PST 2022
sinan updated this revision to Diff 474418.
sinan added a comment.
update the patch according to suggestions.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D136237/new/
https://reviews.llvm.org/D136237
Files:
llvm/lib/CodeGen/BasicBlockSections.cpp
llvm/test/CodeGen/X86/basic-block-sections-redundant-br.ll
Index: llvm/test/CodeGen/X86/basic-block-sections-redundant-br.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/basic-block-sections-redundant-br.ll
@@ -0,0 +1,55 @@
+; Test if basic block sections pass can skip inserting a unconditional branch when a basic block
+; already has an unconditional branch to its fallthrough block.
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=all -O0 | FileCheck %s
+; This test case is generated from code:
+; int
+; mapping (int len)
+; {
+; switch (len)
+; {
+; case 7: return 333;
+; default:
+; goto unknown;
+; }
+; unknown:
+; return 0;
+; }
+; clang -O0 -fbasic-block-sections=all test.c
+
+; Function Attrs: noinline nounwind optnone uwtable
+define dso_local i32 @mapping(i32 noundef %len) {
+; CHECK: mapping:
+; CHECK-NEXT: .cfi_startproc
+; CHECK-NEXT: # %bb.0: # %entry
+; CHECK-NEXT: movl {{.*}}
+; CHECK-NEXT: movl {{.*}}
+; CHECK-NEXT: subl {{.*}}
+; CHECK-NEXT: jne mapping.__part.2
+; CHECK-NEXT: jmp mapping.__part.1
+; CHECK-NOT: jmp
+; CHECK: mapping.__part.1:
+; CHECK: mapping.__part.2:
+entry:
+ %retval = alloca i32, align 4
+ %len.addr = alloca i32, align 4
+ store i32 %len, ptr %len.addr, align 4
+ %0 = load i32, ptr %len.addr, align 4
+ switch i32 %0, label %sw.default [
+ i32 7, label %sw.bb
+ ]
+
+sw.bb: ; preds = %entry
+ store i32 333, ptr %retval, align 4
+ br label %return
+
+sw.default: ; preds = %entry
+ br label %unknown
+
+unknown: ; preds = %sw.default
+ store i32 0, ptr %retval, align 4
+ br label %return
+
+return: ; preds = %unknown, %sw.bb
+ %1 = load i32, ptr %retval, align 4
+ ret i32 %1
+}
Index: llvm/lib/CodeGen/BasicBlockSections.cpp
===================================================================
--- llvm/lib/CodeGen/BasicBlockSections.cpp
+++ llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -128,6 +128,12 @@
"into clusters of basic blocks.",
false, false)
+/// Basic blocks with an explicit unconditional branch to its fallthrough block
+/// do not need an extra unconditional branch.
+static bool hasUncoditionalBranch(const MachineBasicBlock &MBB) {
+ return !MBB.empty() && MBB.back().isUnconditionalBranch();
+}
+
// This function updates and optimizes the branching instructions of every basic
// block in a given function to account for changes in the layout.
static void updateBranches(
@@ -141,10 +147,13 @@
// If this block had a fallthrough before we need an explicit unconditional
// branch to that block if either
// 1- the block ends a section, which means its next block may be
- // reorderd by the linker, or
+ // reorderd by the linker,
// 2- the fallthrough block is not adjacent to the block in the new
- // order.
- if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB))
+ // order, or
+ // 3- the terminator of the block is not an unconditional branch to its
+ // fallthrough.
+ if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB) &&
+ !hasUncoditionalBranch(MBB))
TII->insertUnconditionalBranch(MBB, FTMBB, MBB.findBranchDebugLoc());
// We do not optimize branches for machine basic blocks ending sections, as
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