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[WebAssembly] Fix bug in FixBrTables and use branch analysis utils

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Summary:
This commit fixes a bug in the FixBrTables pass in which an
unconditional branch from the switch header block to the jump table
block was not removed before the blocks were combined. The result was
an invalid CFG in the MachineFunction. This commit also switches from
using bespoke branch analysis and deletion code to using the standard
utilities for the same.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81909
This commit is contained in:
Thomas Lively 2020-06-17 12:34:45 -07:00
parent 9bf0fe84c8
commit 447b230f85
2 changed files with 69 additions and 12 deletions
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32
lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp
View File

@ -52,24 +52,32 @@ MachineBasicBlock *fixBrTable(MachineInstr &MI, MachineBasicBlock *MBB,
// Find the conditional jump to the default target. If it doesn't exist, the
// default target is unreachable anyway, so we can choose anything.
auto JumpMII = --HeaderMBB->end();
while (JumpMII->getOpcode() != WebAssembly::BR_IF &&
JumpMII != HeaderMBB->begin()) {
--JumpMII;
}
if (JumpMII->getOpcode() == WebAssembly::BR_IF) {
// Install the default target and remove the jumps in the header.
auto *DefaultMBB = JumpMII->getOperand(0).getMBB();
assert(DefaultMBB != MBB && "Expected conditional jump to default target");
MI.addOperand(MF, MachineOperand::CreateMBB(DefaultMBB));
HeaderMBB->erase(JumpMII, HeaderMBB->end());
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 2> Cond;
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
TII.analyzeBranch(*HeaderMBB, TBB, FBB, Cond);
// Here are the possible outcomes. '_' is nullptr, `J` is the jump table block
// aka MBB, 'D' is the default block.
//
// TBB | FBB | Meaning
// _ | _ | No default block, header falls through to jump table
// J | _ | No default block, header jumps to the jump table
// D | _ | Header jumps to the default and falls through to the jump table
// D | J | Header jumps to the default and also to the jump table
if (TBB && TBB != MBB) {
// Install the default target.
assert((FBB == nullptr || FBB == MBB) &&
"Expected jump or fallthrough to br_table block");
MI.addOperand(MF, MachineOperand::CreateMBB(TBB));
} else {
// Arbitrarily choose the first jump target as the default.
auto *SomeMBB = MI.getOperand(1).getMBB();
MI.addOperand(MachineOperand::CreateMBB(SomeMBB));
}
// Splice the jump table into the header.
// Remove any branches from the header and splice in the jump table instead
TII.removeBranch(*HeaderMBB, nullptr);
HeaderMBB->splice(HeaderMBB->end(), MBB, MBB->begin(), MBB->end());
// Update CFG to skip the old jump table block. Remove shared successors

49
test/CodeGen/WebAssembly/switch-unreachable-default.ll
View File

@ -36,3 +36,52 @@ bb1:
unreachable:
unreachable
}
; CHECK-LABEL: split:
; CHECK: .functype split (i32) -> ()
; CHECK: block
; CHECK: br_if 0
; CHECK: block
; CHECK: block
; CHECK: br_table {1, 1, 1, 1, 1, 1, 1, 0}
; CHECK: .LBB1_2
; CHECK: end_block
; CHECK: br_table {0, 0, 0}
; CHECK: .LBB1_3
; CHECK: end_block
; CHECK: unreachable
; CHECK: .LBB1_4
; CHECK: end_block
; CHECK: end_function
define void @split(i8 %c) {
entry:
switch i8 %c, label %sw.default [
i8 114, label %return
i8 103, label %sw.bb1
i8 98, label %sw.bb2
i8 97, label %sw.bb3
i8 48, label %sw.bb4
i8 49, label %sw.bb5
]
sw.bb1:
unreachable
sw.bb2:
unreachable
sw.bb3:
unreachable
sw.bb4:
unreachable
sw.bb5:
unreachable
sw.default:
unreachable
return:
ret void
}