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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Nemanja Ivanovic (nemanjai)

<details>
<summary>Changes</summary>

An immediate that isn't a multiple of 16 is meaningless and we should always emit the next higher multiple. Otherwise the value is simply truncated of course which leads to clobbering CSR's on the stack.

---
Full diff: https://github.com/llvm/llvm-project/pull/84935.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVFrameLowering.cpp (+1-1) 
- (modified) llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll (+2-1) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
index 8bac41372b5a83..4f3d915818d1ae 100644
--- a/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
@@ -555,7 +555,7 @@ 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;
   }
diff --git a/llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll b/llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll
index e5c2e0180ee0a6..738d1b07e2210c 100644
--- a/llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll
+++ b/llvm/test/CodeGen/RISCV/zcmp-additional-stack.ll
@@ -3,7 +3,8 @@
 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:    addi sp, sp, 8
 ; RV32-NEXT:    .cfi_def_cfa_offset 24
 ; RV32-NEXT:    .cfi_offset ra, -12
 ; RV32-NEXT:    .cfi_offset s0, -8

``````````

</details>


https://github.com/llvm/llvm-project/pull/84935