[RISCV] Align stack size down to a multiple of 16 before using cm.push/pop. (#86073)

This an alternative to #84935 to fix the miscompile, but not be optimal.

The immediate for cm.push/pop must be a multiple of 16. For RVE, it
might not be. It's not easy to increase the stack size without messing
up cfa directives and maybe other things.

This patch rounds the stack size down to a multiple of 16 before
clamping it to 48. This causes an extra addi to be emitted to handle the
remainder.

Once this commited, I can commit #84989 to add verification for these
instructions being generated with valid offsets.

Author: topperc

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -554,8 +554,10 @@ void RISCVFrameLowering::emitPrologue(MachineFunction &MF,
   if (RVFI->isPushable(MF) && FirstFrameSetup != MBB.end() &&
       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);
+    // stack space. Align the stack size down to a multiple of 16. This is
+    // needed for RVE.
+    // FIXME: Can we increase the stack size to a multiple of 16 instead?
+    uint64_t Spimm = std::min(alignDown(StackSize, 16), (uint64_t)48);
     FirstFrameSetup->getOperand(1).setImm(Spimm);
     StackSize -= Spimm;
   }
@@ -776,8 +778,10 @@ void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
   if (RVFI->isPushable(MF) && MBBI != MBB.end() &&
       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);
+    // space. Align the stack size down to a multiple of 16. This is needed for
+    // RVE.
+    // FIXME: Can we increase the stack size to a multiple of 16 instead?
+    uint64_t Spimm = std::min(alignDown(StackSize, 16), (uint64_t)48);
     MBBI->getOperand(1).setImm(Spimm);
     StackSize -= Spimm;
   }

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}, -16
+; 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
@@ -31,7 +32,8 @@ 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:    addi sp, sp, 8
+; RV32-NEXT:    cm.popret {ra, s0-s1}, 16
 entry:
   br label %while.body