Support for 32-bit mingw-w64 in compiler-rt.

[vadimcn]

Summary

Add _alloca routine, since that's what X86FrameLowering::emitStackProbeCall actually uses to probe stack on Win32.
Also, replace or instructions with test. The latter should be marginally more efficient, as it doesn't write into memory.

[vadimcn]

After playing with this some more - it's no fun linking with gcc-generated libraries, because they actually do use __checkstk_ms, so you have to pull in libgcc as well, and end up with duplicate symbols. Let's add _chkstk as an extra, not as replacement.

[vadimcn]

On second thought, elimination of probe for the last page is not correct - that final push of the return address could land on the next page...
Also, I've figured out a better way to preserve eax.

[vadimcn]

Sorry, one more revision. Apparently chkstk needs triple underscore... <sigh>

[rnk]

Here's how I understand the various functions:

• __chkstk from msvcrt: takes size in RAX, probes all pages, returns preserving all registers.
• __chkstk_ms from mingw: copies __chkstk from msvcrt
• _alloca / __chkstk from mingw: helpers for implementing C alloca() with custom conventions. They trash volatile registers (ECX/RCX), update the SP, and return a new object address in RAX.

I'd rather simplify LLVM's X86FrameLowering to always use __chkstk / __chkstk_ms than add support for __alloca and Mingw __chkstk. It reduces the amount of assembly in compiler-rt, and simplifies LLVM in one step. The downside is that the compiler generates more code around dynamic allocas, which really doesn't matter.

What do you think?

lib/builtins/i386/chkstk.S

22

I guess MSVC uses test, so this is fine.

[vadimcn]

In D14044#275303, @rnk wrote:

Here's how I understand the various functions:

• __chkstk from msvcrt: takes size in RAX, probes all pages, returns preserving all registers.
• __chkstk_ms from mingw: copies __chkstk from msvcrt
• _alloca / __chkstk from mingw: helpers for implementing C alloca() with custom conventions. They trash volatile registers (ECX/RCX), update the SP, and return a new object address in RAX.

I think they trash RAX/EAX as well. That EAX contains a pointer to the allocated space on return is just an accident. I took care to preserve EAX because comments in LLVM's emitStackProbeCall say so; not sure if this is used anywhere in LLVM.

I'd rather simplify LLVM's X86FrameLowering to always use __chkstk / __chkstk_ms than add support for __alloca and Mingw __chkstk. It reduces the amount of assembly in compiler-rt, and simplifies LLVM in one step. The downside is that the compiler generates more code around dynamic allocas, which really doesn't matter.

What do you think?

I personally would be fine with this.
However, the description of builtins module on the compiler-rt page says: "builtins provides full support for the libgcc interfaces on supported targets". If compiler-rt is a drop-in for for libgcc, it should provide all functions that libgcc does. If this is not the case, then linking a binary from a mix of LLVM and GCC objects would require linking both to libgcc and to compiler-rt, which, as I've mentioned above, creates a problem with duplicate symbols.
To resolve this in a principled way, we'd probably need to rename all compiler-rt exports. LLVM would also need a new switch to allow targeting either runtime. A new set of triples for "-llvm" environment? This is a bigger can of worms than I'd like to open. :-)

[rnk]

In D14044#275387, @vadimcn wrote:

However, the description of builtins module on the compiler-rt page says: "builtins provides full support for the libgcc interfaces on supported targets". If compiler-rt is a drop-in for for libgcc, it should provide all functions that libgcc does. If this is not the case, then linking a binary from a mix of LLVM and GCC objects would require linking both to libgcc and to compiler-rt, which, as I've mentioned above, creates a problem with duplicate symbols.
To resolve this in a principled way, we'd probably need to rename all compiler-rt exports. LLVM would also need a new switch to allow targeting either runtime. A new set of triples for "-llvm" environment? This is a bigger can of worms than I'd like to open. :-)

Hm, that's pretty compelling. I can easily imagine someone having a few objects built with GCC and wanting to use compiler-rt.

[vadimcn]

@rnk, Sounds like compiler-rt *is* a drop-in replacement for libgcc, so let's add 64-bit versions of chkstk/alloca for gcc objects.

[rnk]

lgtm

[vadimcn]

@rnk Can you please commit it? Thanks!

[martell]

In D14044#279726, @vadimcn wrote:

@rnk Can you please commit it? Thanks!

Landed in http://reviews.llvm.org/rL251928

[martell]

i386/chkstk.S)
i386/chkstk2.S)

should have been

i386/chkstk.S
i386/chkstk2.S)

and same for x64

fixed in rL251931.

lib/builtins/CMakeLists.txt

     set(i386_SOURCES
         ${i386_SOURCES}
         i386/chkstk.S)
+        i386/chkstk2.S)
   endif()
 
   set(i686_SOURCES

lib/builtins/i386/chkstk.S

         jb     1f
 2:
         sub    $0x1000,%ecx
-        orl    $0,(%ecx)
+        test   %ecx,(%ecx)
         sub    $0x1000,%eax
         cmp    $0x1000,%eax
         ja     2b
 1:
         sub    %eax,%ecx
-        orl    $0,(%ecx)
+        test   %ecx,(%ecx)
         pop    %eax
         pop    %ecx
         ret

lib/builtins/i386/chkstk2.S

+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+
+#include "../assembly.h"
+
+#ifdef __i386__
+
+// _chkstk (_alloca) routine - probe stack between %esp and (%esp-%eax) in 4k increments,
+// then decrement %esp by %eax.  Preserves all registers except %esp and flags.
+// This routine is windows specific
+// http://msdn.microsoft.com/en-us/library/ms648426.aspx
+
+.text
+.balign 4
+DEFINE_COMPILERRT_FUNCTION(_alloca) // _chkstk and _alloca are the same function
+DEFINE_COMPILERRT_FUNCTION(_chkstk)
+        push   %ecx
+        cmp    $0x1000,%eax
+        lea    8(%esp),%ecx     // esp before calling this routine -> ecx
+        jb     1f
+2:
+        sub    $0x1000,%ecx
+        test   %ecx,(%ecx)
+        sub    $0x1000,%eax
+        cmp    $0x1000,%eax
+        ja     2b
+1:
+        sub    %eax,%ecx
+        test   %ecx,(%ecx)
+
+        lea    4(%esp),%eax     // load pointer to the return address into eax
+        mov    %ecx,%esp        // install the new top of stack pointer into esp
+        mov    -4(%eax),%ecx    // restore ecx
+        push   (%eax)           // push return address onto the stack
+        sub    %esp,%eax        // restore the original value in eax
+        ret
+END_COMPILERRT_FUNCTION(_chkstk)
+END_COMPILERRT_FUNCTION(_alloca)
+
+#endif // __i386__

lib/builtins/x86_64/chkstk.S

         jb     1f
 2:
         sub    $0x1000,%rcx
-        orl    $0,(%rcx)
+        test   %rcx,(%rcx)
         sub    $0x1000,%rax
         cmp    $0x1000,%rax
         ja     2b
 1:
         sub    %rax,%rcx
-        orl    $0,(%rcx)
+        test   %rcx,(%rcx)
         pop    %rax
         pop    %rcx
         ret