[AArch64][SME] Support split ZPR and PPR area allocationusers/MacDue/split_pprs

For a while we have supported the `-aarch64-stack-hazard-size=<size>`
option, which adds "hazard padding" between GPRs and FPR/ZPRs. However,
there is currently a hole in this mitigation as PPR and FPR/ZPR accesses
to the same area also cause streaming memory hazards (this is noted by
`-pass-remarks-analysis=sme -aarch64-stack-hazard-remark-size=<val>`),
and the current stack layout places PPRs and ZPRs within the same area.

Which looks like:

------------------------------------  Higher address
| callee-saved gpr registers        |
|---------------------------------- |
| lr,fp  (a.k.a. "frame record")    |
|-----------------------------------| <- fp(=x29)
|   <hazard padding>                |
|-----------------------------------|
| callee-saved fp/simd/SVE regs     |
|-----------------------------------|
|        SVE stack objects          |
|-----------------------------------|
| local variables of fixed size     |
|   <FPR>                           |
|   <hazard padding>                |
|   <GPR>                           |
------------------------------------| <- sp
                                    | Lower address

With this patch the stack (and hazard padding) is rearranged so that
hazard padding is placed between the PPRs and ZPRs rather than within
the (fixed size) callee-save region. Which looks something like this:

------------------------------------  Higher address
| callee-saved gpr registers        |
|---------------------------------- |
| lr,fp  (a.k.a. "frame record")    |
|-----------------------------------| <- fp(=x29)
|        callee-saved PPRs          |
|        PPR stack objects          | (These are SVE predicates)
|-----------------------------------|
|   <hazard padding>                |
|-----------------------------------|
|       callee-saved ZPR regs       | (These are SVE vectors)
|        ZPR stack objects          | Note: FPRs are promoted to ZPRs
|-----------------------------------|
| local variables of fixed size     |
|   <FPR>                           |
|   <hazard padding>                |
|   <GPR>                           |
------------------------------------| <- sp
                                    | Lower address

This layout is only enabled if:

 * SplitSVEObjects are enabled (`-aarch64-split-sve-objects`)
   - (This may be enabled by default in a later patch)
 * Streaming memory hazards are present
   - (`-aarch64-stack-hazard-size=<val>` != 0)
 * PPRs and FPRs/ZPRs are on the stack
 * There's no stack realignment or variable-sized objects
   - This is left as a TODO for now

Additionally, any FPR callee-saves that are present will be promoted to
ZPRs. This is to prevent stack hazards between FPRs and GRPs in the
fixed size callee-save area (which would otherwise require more hazard
padding, or moving the FPR callee-saves).

This layout should resolve the hole in the hazard padding mitigation,
and is not intended change codegen for non-SME code.

Change-Id: I2e1906577c2ac79c40bc69e7c15e3ef09857445f

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h b/llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h
index 4a79d9c..096e180 100644
--- a/llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h
+++ b/llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h
@@ -137,6 +137,10 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
   uint64_t StackSizeZPR = 0;
   uint64_t StackSizePPR = 0;
 
+  /// Are SVE objects (vectors and predicates) split into separate regions on
+  /// the stack.
+  bool SplitSVEObjects = false;
+
   /// HasCalculatedStackSizeSVE indicates whether StackSizeZPR/PPR is valid.
   bool HasCalculatedStackSizeSVE = false;
 
@@ -336,7 +340,6 @@ public:
 
   bool isStackRealigned() const { return StackRealigned; }
   void setStackRealigned(bool s) { StackRealigned = s; }
-
   bool hasCalleeSaveStackFreeSpace() const {
     return CalleeSaveStackHasFreeSpace;
   }
@@ -481,6 +484,9 @@ public:
     StackHazardCSRSlotIndex = Index;
   }
 
+  bool hasSplitSVEObjects() const { return SplitSVEObjects; }
+  void setSplitSVEObjects(bool s) { SplitSVEObjects = s; }
+
   SMEAttrs getSMEFnAttrs() const { return SMEFnAttrs; }
 
   unsigned getSRetReturnReg() const { return SRetReturnReg; }