[PowerPC] Fix wrong codegen when stack pointer has to realign in prologue

[lkail]

Summary

Current powerpc backend generates wrong code sequence if stack pointer has to realign if -fstack-clash-protection enabled. When probing in prologue, backend should generate a subtraction instruction rather than a stux instruction to realign the stack pointer.

This patch is part of fix of https://bugs.llvm.org/show_bug.cgi?id=46759

[jsji]

LGTM.

[hans]

Was I cc'd because you want this cherry-picked to the 11.x branch?

[lkail]

In D84218#2166788, @hans wrote:

Was I cc'd because you want this cherry-picked to the 11.x branch?

Yes. I've open https://bugs.llvm.org/show_bug.cgi?id=46759 and set it as release blocker to track the issue.

[hans]

Great, I'll pick it up there. I'm reading my email in the wrong order today it seems :)

llvm/lib/Target/PowerPC/PPCFrameLowering.cpp

if (isPPC64)
           .addImm(0)
           .addImm(64 - Log2(MaxAlign));
     else
       BuildMI(PrologMBB, {MI}, DL, TII.get(PPC::RLWINM), ScratchReg)
           .addReg(FPReg)
           .addImm(0)
           .addImm(32 - Log2(MaxAlign))
           .addImm(31);
-    BuildMI(PrologMBB, {MI}, DL, TII.get(isPPC64 ? PPC::STDUX : PPC::STWUX),
+    BuildMI(PrologMBB, {MI}, DL, TII.get(isPPC64 ? PPC::SUBFC8 : PPC::SUBFC),
             SPReg)
-        .addReg(FPReg)
-        .addReg(SPReg)
-        .addReg(ScratchReg);
+        .addReg(ScratchReg)
+        .addReg(SPReg);
   }
   // Probe residual part.
   if (NegResidualSize) {
     bool ResidualUseDForm = CanUseDForm(NegResidualSize);
     if (!ResidualUseDForm)
       MaterializeImm(PrologMBB, {MI}, NegResidualSize, ScratchReg);
     allocateAndProbe(PrologMBB, {MI}, NegResidualSize, ScratchReg,
                      ResidualUseDForm);

llvm/test/CodeGen/PowerPC/pr46759.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -ppc-asm-full-reg-names -verify-machineinstrs \
 ; RUN:   -mtriple=powerpc64le-linux-gnu < %s | FileCheck \
 ; RUN:   -check-prefix=CHECK-LE %s
 
 define void @foo(i32 %vla_size) #0 {
 ; CHECK-LE-LABEL: foo:
 ; CHECK-LE:       # %bb.0: # %entry
 ; CHECK-LE-NEXT:    std r31, -8(r1)
 ; CHECK-LE-NEXT:    std r30, -16(r1)
 ; CHECK-LE-NEXT:    mr r30, r1
 ; CHECK-LE-NEXT:    mr r12, r1
 ; CHECK-LE-NEXT:    .cfi_def_cfa r12, 0
 ; CHECK-LE-NEXT:    clrldi r0, r12, 53
-; CHECK-LE-NEXT:    stdux r12, r1, r0
+; CHECK-LE-NEXT:    subc r1, r1, r0
 ; CHECK-LE-NEXT:    stdu r12, -2048(r1)
 ; CHECK-LE-NEXT:    stdu r12, -4096(r1)
 ; CHECK-LE-NEXT:    .cfi_def_cfa_register r1
 ; CHECK-LE-NEXT:    .cfi_def_cfa_register r30
 ; CHECK-LE-NEXT:    .cfi_offset r31, -8
 ; CHECK-LE-NEXT:    .cfi_offset r30, -16
 ; CHECK-LE-NEXT:    clrldi r3, r3, 32
 ; CHECK-LE-NEXT:    li r6, -4096