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Stack reservation is not optimal #9

Open kv-sc opened 5 months ago

kv-sc commented 5 months ago

reproduce on 1.0

snippy invocation line:

./llvm-snippy layout-stack.yaml -march=riscv64-linux-gnu -function-number=3 -function-layers=2 -num-instrs=100  -stack-size=10240 -num-instr-ancil=50 -model-plugin=None

layout-stack.yaml looks like this:

sections:
  - no:        1
    VMA:       0x210000
    SIZE:      0x100000
    LMA:       0x210000
    ACCESS:    rx
  - no:        2
    VMA:       0x100000
    SIZE:      0x100000
    LMA:       0x100000
    ACCESS:    rw

histogram:
    - [ADD, 1.0]
    - [ADDI, 1.0]
    - [SUB, 1.0]
    - [JAL, 0.1]
    - [JALR, 0.1]

You will see in generated binary:

000000000000043c <fun1>:
 43c:   ff010113                addi    sp,sp,-16
 440:   01813023                sd      s8,0(sp)
 444:   ff010113                addi    sp,sp,-16
 448:   01f13023                sd      t6,0(sp)
 44c:   ff010113                addi    sp,sp,-16
 450:   00c13023                sd      a2,0(sp)
 454:   ff010113                addi    sp,sp,-16
 458:   01a13023                sd      s10,0(sp)
 45c:   ff010113                addi    sp,sp,-16
 460:   00713023                sd      t2,0(sp)
 464:   ff010113                addi    sp,sp,-16
 468:   00e13023                sd      a4,0(sp)
....

Granted that there is only 2 layers of functions in call graph it is possible to statically assign space for each function on stack and calculate it from scratch in the prologue.