bug fix for PR20020: anti-dependency-breaker causes miscompilation
This patch sets the 'KeepReg' bit for any tied and live registers during the PrescanInstruction() phase of the dependency breaking algorithm. It then checks those 'KeepReg' bits during the ScanInstruction() phase to avoid changing any tied registers. For more details, please see comments in:
http://llvm.org/bugs/show_bug.cgi?id=20020
I added two FIXME comments for code that I think can be removed by using register iterators that include self. I don't want to include those code changes with this patch, however, to keep things as small as possible.
The test case is larger than I'd like, but I don't know how to reduce it further and still produce the failing asm.
Differential Revision: http://reviews.llvm.org/D4351
llvm-svn: 212275
diff --git a/llvm/lib/CodeGen/CriticalAntiDepBreaker.cpp b/llvm/lib/CodeGen/CriticalAntiDepBreaker.cpp
index 3e0f740..d3ffcc7 100644
--- a/llvm/lib/CodeGen/CriticalAntiDepBreaker.cpp
+++ b/llvm/lib/CodeGen/CriticalAntiDepBreaker.cpp
@@ -200,6 +200,28 @@
if (Classes[Reg] != reinterpret_cast<TargetRegisterClass *>(-1))
RegRefs.insert(std::make_pair(Reg, &MO));
+ // If this reg is tied and live (Classes[Reg] is set to -1), we can't change
+ // it or any of its sub or super regs. We need to use KeepRegs to mark the
+ // reg because not all uses of the same reg within an instruction are
+ // necessarily tagged as tied.
+ // Example: an x86 "xor %eax, %eax" will have one source operand tied to the
+ // def register but not the second (see PR20020 for details).
+ // FIXME: can this check be relaxed to account for undef uses
+ // of a register? In the above 'xor' example, the uses of %eax are undef, so
+ // earlier instructions could still replace %eax even though the 'xor'
+ // itself can't be changed.
+ if (MI->isRegTiedToUseOperand(i) &&
+ Classes[Reg] == reinterpret_cast<TargetRegisterClass *>(-1)) {
+ for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
+ SubRegs.isValid(); ++SubRegs) {
+ KeepRegs.set(*SubRegs);
+ }
+ for (MCSuperRegIterator SuperRegs(Reg, TRI);
+ SuperRegs.isValid(); ++SuperRegs) {
+ KeepRegs.set(*SuperRegs);
+ }
+ }
+
if (MO.isUse() && Special) {
if (!KeepRegs.test(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
@@ -236,9 +258,15 @@
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
if (!MO.isDef()) continue;
+
+ // If we've already marked this reg as unchangeable, carry on.
+ if (KeepRegs.test(Reg)) continue;
+
// Ignore two-addr defs.
if (MI->isRegTiedToUseOperand(i)) continue;
+ // FIXME: we should use a SubRegIterator that includes self (as above), so
+ // we don't have to repeat all this code for the reg itself.
DefIndices[Reg] = Count;
KillIndices[Reg] = ~0u;
assert(((KillIndices[Reg] == ~0u) !=
@@ -281,6 +309,9 @@
RegRefs.insert(std::make_pair(Reg, &MO));
+ // FIXME: we should use an MCRegAliasIterator that includes self so we don't
+ // have to repeat all this code for the reg itself.
+
// It wasn't previously live but now it is, this is a kill.
if (KillIndices[Reg] == ~0u) {
KillIndices[Reg] = Count;