start: Avoid going through the argc_argv_ptr global variable
#20718
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This is problematic for PIE. There's nothing but luck preventing the accesses to this global variable from requiring relocations. I've observed this being an issue on MIPS and PowerPC personally, but others may be affected. Besides, we're really just passing the initial stack pointer value to posixCallMainAndExit(), so... just do that.
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Regresses PPC64, marking as draft temporarily. |
The previous version of this function referenced the argc_argv_ptr global variable as an inline asm operand. This caused LLVM to generate prologue code to initialize the ToC so that the global variable can actually be accessed. Ordinarily, there's nothing wrong with that. But _start() is a naked function! This makes it actually super surprising that LLVM did this. It also means that the old version only really worked by accident. Once the reference to the global variable was removed, no ToC was set up, thus violating the calling convention once we got to posixCallMainAndExit(). This then caused any attempt to access global variables here to crash - namely when setting std.os.linux.elf_aux_maybe. The fix is to just initialize the ToC manually in _start().
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Ok, PPC64 is working again. Turns out the old code only worked by accident and my changes exposed that fact. |
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Nice work. I remember why I originally made it write to a global variable. It used to be that an optimized "hello world" or other minimal program would be equivalent to the hand-crafted assembly. Now that can no longer be the case because there will always be a call to posixCallMainAndExit due to the inline asm. It's not a really important use case, but was nice and now that beauty is lost. Not with your patch, which is a strict improvement, but with the patch that added the function call to the inline asm. |
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This is problematic for PIE. There's nothing but luck preventing the accesses to this global variable from requiring relocations. I've observed this being an issue on MIPS and PowerPC personally, but others may be affected.