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[RISC-V] Only emit multiples of 16 as immediate for cm.push #84935

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[RISC-V] Only emit multiples of 16 as immediate for cm.push #84935

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f6efe6f
[RISC-V] Only emit multiples of 16 as immediate for cm.push
Mar 12, 2024
7a5cb38
Remove the unnecessary stack adjustment (as it's wrong).
Mar 12, 2024
a22b64e
Fix cm.popret as well...
Mar 13, 2024
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8 changes: 4 additions & 4 deletions llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -555,9 +555,9 @@ void RISCVFrameLowering::emitPrologue(MachineFunction &MF,
FirstFrameSetup->getOpcode() == RISCV::CM_PUSH) {
// Use available stack adjustment in push instruction to allocate additional
// stack space.
uint64_t Spimm = std::min(StackSize, (uint64_t)48);
uint64_t Spimm = alignTo(std::min(StackSize, (uint64_t)48), 16);
FirstFrameSetup->getOperand(1).setImm(Spimm);
StackSize -= Spimm;
StackSize -= std::min(StackSize, Spimm);
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Does this mean we can move the sp by more than StackSize? Doesn't that make the position of sp out of sync with the rest of the code?

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Well, if the amount of space in the offset for the cm.push is more than the additional stack space required, then yes we could. I don't follow why this is a problem though. Or maybe I am misunderstanding what the StackSize variable actually represents - the way I have interpreted it is "The amount of stack space required for parameters and local objects (not including the space to spill CSR's)". Of course, if my interpretation is correct, I'd certainly make the argument that the variable is poorly named, but hey.

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We can move the sp by more than StackSize here, and if it does, the stack pointer will be readjusted in the following if statement where StackSize != 0.

}

if (StackSize != 0) {
Expand Down Expand Up @@ -777,9 +777,9 @@ void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
MBBI->getOpcode() == RISCV::CM_POP) {
// Use available stack adjustment in pop instruction to deallocate stack
// space.
uint64_t Spimm = std::min(StackSize, (uint64_t)48);
uint64_t Spimm = alignTo(std::min(StackSize, (uint64_t)48), 16);
MBBI->getOperand(1).setImm(Spimm);
StackSize -= Spimm;
StackSize -= std::min(StackSize, Spimm);
}

// Deallocate stack
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4 changes: 2 additions & 2 deletions llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
define ptr @func(ptr %s, i32 %_c, ptr %incdec.ptr, i1 %0, i8 %conv14) #0 {
; RV32-LABEL: func:
; RV32: # %bb.0: # %entry
; RV32-NEXT: cm.push {ra, s0-s1}, -24
; RV32-NEXT: cm.push {ra, s0-s1}, -32
; RV32-NEXT: .cfi_def_cfa_offset 24
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Aren't these cfi directives wrong, since the stack pointer was moved by 32 not 24?

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They most certainly appear to be wrong.

; RV32-NEXT: .cfi_offset ra, -12
; RV32-NEXT: .cfi_offset s0, -8
Expand Down Expand Up @@ -31,7 +31,7 @@ define ptr @func(ptr %s, i32 %_c, ptr %incdec.ptr, i1 %0, i8 %conv14) #0 {
; RV32-NEXT: lw a0, 4(sp) # 4-byte Folded Reload
; RV32-NEXT: sb a0, 0(s0)
; RV32-NEXT: mv a0, s1
; RV32-NEXT: cm.popret {ra, s0-s1}, 24
; RV32-NEXT: cm.popret {ra, s0-s1}, 32
entry:
br label %while.body

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