MdeModulePkg/EbcDxe AARCH64: use tail call for EBC to native thunk
Instead of pessimistically copying at least 64 bytes from the VM stack
to the native stack, and popping off the register arguments again
before doing the native call, try to avoid touching the stack completely
if the VM stack frame is <= 64 bytes. Also, if the stack frame does exceed
64 bytes, there is no need to copy the first 64 bytes, since we are passing
those in registers anyway.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Feng Tian <feng.tian@intel.com>
diff --git a/MdeModulePkg/Universal/EbcDxe/AArch64/EbcLowLevel.S b/MdeModulePkg/Universal/EbcDxe/AArch64/EbcLowLevel.S
index b4b8531..34794c0 100644
--- a/MdeModulePkg/Universal/EbcDxe/AArch64/EbcLowLevel.S
+++ b/MdeModulePkg/Universal/EbcDxe/AArch64/EbcLowLevel.S
@@ -35,30 +35,75 @@
//****************************************************************************
// UINTN EbcLLCALLEXNative(UINTN FuncAddr, UINTN NewStackPointer, VOID *FramePtr)
ASM_PFX(EbcLLCALLEXNative):
- stp x19, x20, [sp, #-16]!
- stp x29, x30, [sp, #-16]!
+ mov x8, x0 // Preserve x0
+ mov x9, x1 // Preserve x1
- mov x19, x0
- mov x20, sp
- sub x2, x2, x1 // Length = NewStackPointer-FramePtr
- sub sp, sp, x2
- sub sp, sp, #64 // Make sure there is room for at least 8 args in the new stack
- mov x0, sp
+ //
+ // If the EBC stack frame is smaller than or equal to 64 bytes, we know there
+ // are no stacked arguments #9 and beyond that we need to copy to the native
+ // stack. In this case, we can perform a tail call which is much more
+ // efficient, since there is no need to touch the native stack at all.
+ //
+ sub x3, x2, x1 // Length = NewStackPointer - FramePtr
+ cmp x3, #64
+ b.gt 1f
- bl CopyMem // Sp, NewStackPointer, Length
+ //
+ // While probably harmless in practice, we should not access the VM stack
+ // outside of the interval [NewStackPointer, FramePtr), which means we
+ // should not blindly fill all 8 argument registers with VM stack data.
+ // So instead, calculate how many argument registers we can fill based on
+ // the size of the VM stack frame, and skip the remaining ones.
+ //
+ adr x0, 0f // Take address of 'br' instruction below
+ bic x3, x3, #7 // Ensure correct alignment
+ sub x0, x0, x3, lsr #1 // Subtract 4 bytes for each arg to unstack
+ br x0 // Skip remaining argument registers
- ldp x0, x1, [sp], #16
- ldp x2, x3, [sp], #16
- ldp x4, x5, [sp], #16
- ldp x6, x7, [sp], #16
+ ldr x7, [x9, #56] // Call with 8 arguments
+ ldr x6, [x9, #48] // |
+ ldr x5, [x9, #40] // |
+ ldr x4, [x9, #32] // |
+ ldr x3, [x9, #24] // |
+ ldr x2, [x9, #16] // |
+ ldr x1, [x9, #8] // V
+ ldr x0, [x9] // Call with 1 argument
- blr x19
+0: br x8 // Call with no arguments
- mov sp, x20
- ldp x29, x30, [sp], #16
- ldp x19, x20, [sp], #16
+ //
+ // More than 64 bytes: we need to build the full native stack frame and copy
+ // the part of the VM stack exceeding 64 bytes (which may contain stacked
+ // arguments) to the native stack
+ //
+1: stp x29, x30, [sp, #-16]!
+ mov x29, sp
- ret
+ //
+ // Ensure that the stack pointer remains 16 byte aligned,
+ // even if the size of the VM stack frame is not a multiple of 16
+ //
+ add x1, x1, #64 // Skip over [potential] reg params
+ tbz x3, #3, 2f // Multiple of 16?
+ ldr x4, [x2, #-8]! // No? Then push one word
+ str x4, [sp, #-16]! // ... but use two slots
+ b 3f
+
+2: ldp x4, x5, [x2, #-16]!
+ stp x4, x5, [sp, #-16]!
+3: cmp x2, x1
+ b.gt 2b
+
+ ldp x0, x1, [x9]
+ ldp x2, x3, [x9, #16]
+ ldp x4, x5, [x9, #32]
+ ldp x6, x7, [x9, #48]
+
+ blr x8
+
+ mov sp, x29
+ ldp x29, x30, [sp], #16
+ ret
//****************************************************************************
// EbcLLEbcInterpret
