O3: Fix itstate prediction and recovery.

Any change of control flow now resets the itstate to 0 mask and 0 condition,
except where the control flow alteration write into the cpsr register. These
case, for example return from an iterrupt, require the predecoder to recover
the itstate.

As there is a window of opportunity between the return from an interrupt
changing the control flow at the head of the pipe and the commit of the update
to the CPSR, the predecoder needs to be able to grab the ITstate early. This
is now handled by setting the forcedItState inside a PCstate for the control
flow altering instruction.

That instruction will have the correct mask/cond, but will not have a valid
itstate until advancePC is called (note this happens to advance the execution).
When the new PCstate is copy constructed it gets the itstate cond/mask, and
upon advancing the PC the itstate becomes valid.

Subsequent advancing invalidates the state and zeroes the cond/mask. This is
handled in isolation for the ARM ISA and should have no impact on other ISAs.

Refer arch/arm/types.hh and arch/arm/predecoder.cc for the details.

1 files changed, 11 insertions, 3 deletions

diff --git a/src/arch/arm/predecoder.cc b/src/arch/arm/predecoder.cc
index 71d399e35..e532359b5 100644
--- a/src/arch/arm/predecoder.cc
+++ b/src/arch/arm/predecoder.cc
@@ -162,9 +162,17 @@ Predecoder::moreBytes(const PCState &pc, Addr fetchPC, MachInst inst)
     FPSCR fpscr = tc->readMiscReg(MISCREG_FPSCR);
     emi.fpscrLen = fpscr.len;
     emi.fpscrStride = fpscr.stride;
-    CPSR cpsr = tc->readMiscReg(MISCREG_CPSR);
-    itstate.top6 = cpsr.it2;
-    itstate.bottom2 = cpsr.it1;
+
+    if (pc.forcedItStateIsValid()) {
+        // returns from exceptions/interrupts force the it state.
+        itstate = pc.forcedItState();
+        DPRINTF(Predecoder, "Predecoder, itstate forced = %08x.\n", pc.forcedItState());
+    } else if (predAddrValid && (pc.instAddr() != predAddr)) {
+        // Control flow changes necessitate a 0 itstate.
+        itstate.top6 = 0;
+        itstate.bottom2 = 0;
+    }
+
     outOfBytes = false;
     process();
 }