[PATCH] D109203: [X86] Don't clobber EBX in stackprobes

[Elliot Saba via Phabricator via llvm-commits]

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On X86, the stackprobe emission code chooses the `R11D` register, which
is illegal on i686.  This ends up wrapping around to `EBX`, which does
not get properly callee-saved within the stack probing prologue,
clobbering the register for the callers.

We fix this by explicitly using `EAX` as the stack probe register.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D109203

Files:
  llvm/lib/Target/X86/X86FrameLowering.cpp


Index: llvm/lib/Target/X86/X86FrameLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86FrameLowering.cpp
+++ llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -652,7 +652,7 @@
   MF.insert(MBBIter, testMBB);
   MF.insert(MBBIter, tailMBB);
 
-  Register FinalStackProbed = Uses64BitFramePtr ? X86::R11 : X86::R11D;
+  Register FinalStackProbed = Uses64BitFramePtr ? X86::R11 : X86::EAX;
   BuildMI(MBB, MBBI, DL, TII.get(TargetOpcode::COPY), FinalStackProbed)
       .addReg(StackPtr)
       .setMIFlag(MachineInstr::FrameSetup);
@@ -1073,7 +1073,7 @@
       MF.insert(MBBIter, bodyMBB);
       MF.insert(MBBIter, footMBB);
       const unsigned MovMIOpc = Is64Bit ? X86::MOV64mi32 : X86::MOV32mi;
-      Register FinalStackProbed = Uses64BitFramePtr ? X86::R11 : X86::R11D;
+      Register FinalStackProbed = Uses64BitFramePtr ? X86::R11 : X86::EAX;
 
       // Setup entry block
       {


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