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[x86/SLH] Extract another small helper function, add better comments and 

use better terminology. NFC.

llvm-svn: 337157
This commit is contained in:
Chandler Carruth 2018-07-16 10:46:16 +00:00
parent f4e7025299
commit 3620b9957a
1 changed files with 34 additions and 23 deletions
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57
llvm/lib/Target/X86/X86SpeculativeLoadHardening.cpp
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@ -330,6 +330,37 @@ static void canonicalizePHIOperands(MachineFunction &MF) {
}
}
/// Helper to scan a function for loads vulnerable to misspeculation that we
/// want to harden.
///
/// We use this to avoid making changes to functions where there is nothing we
/// need to do to harden against misspeculation.
static bool hasVulnerableLoad(MachineFunction &MF) {
for (MachineBasicBlock &MBB : MF) {
for (MachineInstr &MI : MBB) {
// Loads within this basic block after an LFENCE are not at risk of
// speculatively executing with invalid predicates from prior control
// flow. So break out of this block but continue scanning the function.
if (MI.getOpcode() == X86::LFENCE)
break;
// Looking for loads only.
if (!MI.mayLoad())
continue;
// An MFENCE is modeled as a load but isn't vulnerable to misspeculation.
if (MI.getOpcode() == X86::MFENCE)
continue;
// We found a load.
return true;
}
}
// No loads found.
return false;
}
bool X86SpeculativeLoadHardeningPass::runOnMachineFunction(
MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** " << getPassName() << " : " << MF.getName()
@ -359,34 +390,14 @@ bool X86SpeculativeLoadHardeningPass::runOnMachineFunction(
auto EntryInsertPt = Entry.SkipPHIsLabelsAndDebug(Entry.begin());
// Do a quick scan to see if we have any checkable loads.
bool HasCheckableLoad = false;
for (MachineBasicBlock &MBB : MF) {
for (MachineInstr &MI : MBB) {
// Stop searching blocks at an LFENCE.
if (MI.getOpcode() == X86::LFENCE)
break;
// Looking for loads only.
if (!MI.mayLoad())
continue;
// An MFENCE is modeled as a load but doesn't require hardening.
if (MI.getOpcode() == X86::MFENCE)
continue;
HasCheckableLoad = true;
break;
}
if (HasCheckableLoad)
break;
}
bool HasVulnerableLoad = hasVulnerableLoad(MF);
// See if we have any conditional branching blocks that we will need to trace
// predicate state through.
SmallVector<BlockCondInfo, 16> Infos = collectBlockCondInfo(MF);
// If we have no interesting conditions or loads, nothing to do here.
if (!HasCheckableLoad && Infos.empty())
if (!HasVulnerableLoad && Infos.empty())
return true;
unsigned PredStateReg;
@ -402,7 +413,7 @@ bool X86SpeculativeLoadHardeningPass::runOnMachineFunction(
// If we have loads being hardened and we've asked for call and ret edges to
// get a full fence-based mitigation, inject that fence.
if (HasCheckableLoad && FenceCallAndRet) {
if (HasVulnerableLoad && FenceCallAndRet) {
// We need to insert an LFENCE at the start of the function to suspend any
// incoming misspeculation from the caller. This helps two-fold: the caller
// may not have been protected as this code has been, and this code gets to