I've submitted 207175 for a clang 3.8.0 va_list handling problem for powerpc

Mon Feb 15 20:17:58 UTC 2016

On 2016-Feb-15, at 11:11 AM, Roman Divacky <rdivacky at vlakno.cz> wrote:
> 
> Mark, I believe you're right. What do you think about this patch?
> 
> Index: tools/clang/lib/CodeGen/TargetInfo.cpp
> ===================================================================
> --- tools/clang/lib/CodeGen/TargetInfo.cpp	(revision 260852)
> +++ tools/clang/lib/CodeGen/TargetInfo.cpp	(working copy)
> @@ -3599,6 +3599,8 @@
>   {
>     CGF.EmitBlock(UsingOverflow);
> 
> +    Builder.CreateStore(Builder.getInt8(11), NumRegsAddr);
> +
>     // Everything in the overflow area is rounded up to a size of at least 4.
>     CharUnits OverflowAreaAlign = CharUnits::fromQuantity(4);
> 
> 
> Can you test it?

It may be later today before I can start the the test process.

While your change is not wrong as presented, it does seem to be based on the ABI document's numbering with the range 3 <= gr <12, where 3 <= gr < 11 cover r3-r10 use and gr=11 implies overflow stack area use. (gr being the ABI documents name.)

The clang code generation that I saw while analyzing the problem and the clang source that you had me look at did not use that numbering. Instead it seems to be based on 0 <= gpr < 9, where 0 <= gpr < 8 cover r3-r10 use and gpr=8 implies overflow stack area use. (gpr being what gdb showed me as I remember.) In other words: clang counts the number of "parameter registers" already in use as it goes along instead of tracking register numbers that have been used.

So assigning any value that appears to be positive and >= 8 should work, such as:

Builder.CreateStore(Builder.getInt8(8), NumRegsAddr);

A cross check on this is the clang source code below:

> llvm::Value *NumRegs = Builder.CreateLoad(NumRegsAddr, "numUsedRegs");
> . . .
> llvm::Value *CC =
>     Builder.CreateICmpULT(NumRegs, Builder.getInt8(8), "cond");
> 
> llvm::BasicBlock *UsingRegs = CGF.createBasicBlock("using_regs");
> llvm::BasicBlock *UsingOverflow = CGF.createBasicBlock("using_overflow");
> llvm::BasicBlock *Cont = CGF.createBasicBlock("cont");
> 
> Builder.CreateCondBr(CC, UsingRegs, UsingOverflow);

I'd guess that the Builder.CreateICmpULT(NumRegs, Builder.getInt8(8), "cond") for using in Builder.CreateCondBr is a test for < 8 (unsigned test?) picking UsingRegs and >=8 picking UsingOverflow. 11>=8 so 11 would work.

But the clang folks might prefer that the same figure be used in both places, possibly with the source code naming the value once and using the name in both places, not that the figure is likely to change in this already PowerPC specific code.

In analyzing the powerpc code absent knowledge of clang's code generation source code I would likely have been confused by seeing such differing numbers in the generated code if I'd run into such. That is another reason to use the same figure in both places.

I continue to provide some history below for reference.

===
Mark Millard
markmi at dsl-only.net

On Mon, Feb 15, 2016 at 12:52:15AM -0800, Mark Millard wrote:
> 
> I'm top posting as the following can stand on its own fairly well.
> 
> On Sun Feb 14 23:46:14 UTC 2016 Nathan Whitehorn wrote:
> 
>> On 02/14/16 14:34, Mark Millard wrote:
>>> clang's code base is not familiar material for me nor do I have solid 
>>> reference material for the FreeBSD TARGET_ARCH=powerpc ABI rules so 
>>> the below has my guess work involved. The following code appears to 
>>> have hard wired a global, unvarying constant (8) into the test for 
>>> picking UsingRegs vs. UsingOverflow.
>> 
>> For reference, we use the standard ELF ABI 
>> (https://uclibc.org/docs/psABI-ppc.pdf).
>> -Nathan
> 
> Reviewing the Parameter Passing material in that document shows that the problem is in the original specification.
> 
> And there is a more modern specification that has a fix in its wording. (Which shows that I'm not likely to be wrong.) I'll reference and quote it later.
> 
> First I'll explain the problem that is in psABI-ppc.pdf (the old SunSoft 1995 document).
> 
> First a numbering point: psABI-ppc.pdf uses "gr" matching the numeral in r3, r4, . . . , r10, starting at r3 (i.e, 3). And gr indicates the next register to be used, not the last one already used.
> 
> The document splits the algorithm for placement of parameters into 3 stages with the following structure, intended as they have it in the document but various less interesting details for my "8byte then 4byte" example omitted:
> 
>> INITIALIZING:
>>     Set fr=1, gr=3, and starg to the address of
>>     parameter word 1.
>> SCAN:
>>     If there are no more arguments, terminate.
>>     Otherwise, select one of the following
>>     depending on the type of the next argument:
>> 
>>     DOUBLE_OR_FLOAT
>>        If fr>8 ( . . .), go to OTHER. Otherwise,
>>        . . .
>> 
>>     SIMPLE_ARG
>>        If gr>10, go to OTHER. Otherwise, load the
>>        argument value into general register gr,
>>        set gr to gr+1, can goto SCAN. . . .
>> 
>>     LONG_LONG
>>        If gr>9, go to OTHER. Otherwise, . . .
>> 
>> OTHER:
>>       Arguments not otherwise handled above are
>>       passed in the parameter words of the
>>       caller???s stack frame. . . . Set starg to
>>       starg+size, then go to SCAN.
> 
> Note that gr is not incremented by LONG_LONG or by the later OTHER usage when gr>9. (That would be my example's 8 byte integer that is later followed by a 4 byte one.)
> 
> That OTHER's "go to SCAN" would then lead to the following 4 byte integer in my example to be put in r10 and gr then being set to 11 instead of it being stored in a parameter word on the stack.
> 
> The nasty thing about this for va_list/va_arg use is that the stored information does not indicate which was before vs. after in the argument order: the 4 byte r10 content or the 8 byte "OTHER" content: the two orders produce identical results.
> 
> This can not be correct.
> 
> The Power-Arch-32-bit-ABI-supp-1.0-Unified.pdf is more modern and explicitly deals with VR and other modern things. (Its terminology matching LONG_LONG above is DUAL_GP.) But for what I'm dealing with here it has the following extra wording at the very end of its OTHER section:
> 
>> If gr>9 and the type is DUAL_GP ,or . . ., or . . ., then set gr = 11 (to prevent subsequent SINGLE_GPs from being placed in registers after DUAL_GP, QUAD_GP, or EIGHT_GP arguments that would no longer fit in the registers).
> 
> 
> 
> I've left the prior information below for reference.
> 
> ===
> Mark Millard
> markmi at dsl-only.net
> 
> 
> 
> On 2016-Feb-14, at 2:34 PM, Mark Millard <markmi at dsl-only.net> wrote:
>> 
>> . . .
>> clang's code base is not familiar material for me nor do I have solid reference material for the FreeBSD TARGET_ARCH=powerpc ABI rules so the below has my guess work involved.
>> 
>> The following code appears to have hard wired a global, unvarying constant (8) into the test for picking UsingRegs vs. UsingOverflow.
>> 
>> 
>>> llvm::Value *NumRegs = Builder.CreateLoad(NumRegsAddr, "numUsedRegs");
>> . . .
>>> llvm::Value *CC =
>>>     Builder.CreateICmpULT(NumRegs, Builder.getInt8(8), "cond");
>>> 
>>> llvm::BasicBlock *UsingRegs = CGF.createBasicBlock("using_regs");
>>> llvm::BasicBlock *UsingOverflow = CGF.createBasicBlock("using_overflow");
>>> llvm::BasicBlock *Cont = CGF.createBasicBlock("cont");
>>> 
>>> Builder.CreateCondBr(CC, UsingRegs, UsingOverflow);
>> . . .
>>> // Case 1: consume registers.
>>> Address RegAddr = Address::invalid();
>>> {
>> . . .
>>>   // Increase the used-register count.
>>>   NumRegs =
>>>     Builder.CreateAdd(NumRegs,
>>>                       Builder.getInt8((isI64 || (isF64 && IsSoftFloatABI)) ? 2 : 1));
>>>   Builder.CreateStore(NumRegs, NumRegsAddr);. . .
>> . . .
>>> }
>>> 
>>> // Case 2: consume space in the overflow area.
>>> Address MemAddr = Address::invalid();
>>> {
>> . . . (no adjustments to NumRegs) . . .
>> 
>> If so the means of counting NumRegs (a.k.a. gpr) then needs to take into account an allocated but unused last UsingRegs "slot" sometimes. Imagine. . .
>> 
>> r3, r4, r5, r6, r7, r8, r9 in use already so r10 is the last possible "UsingRegs" context.
>> (0  1   2   3   4   5   6, leaving r10 as position 7, the last < 8 value)
>> 
>> Then the next two arguments are a 8 byte integer then a a 4 byte integer (in that order). That results in what should be:
>> 
>> r10 "UsingRegs" slot reserved and un-accessed
>> In other words: counted as allocated so that the rest goes in in the overflow area
>> (so no position 7 usage)
>> 
>> then
>> 
>> overflow with the 8 byte integer then the 4 byte integer.
>> 
>> 
>> And, in fact, the memory content reflects this in the overflow area.
>> 
>> 
>> But the va_arg access code does not count r10's slot as allocated in "Using Regs" after the 8 byte integer. So later it tries to use r10's slot for the 4 byte integer that is actually in the UsingOverflow area.
>> 
>> One fix of sorts is to have "Case 2: consume space in the overflow area." set NumRegs (a.k.a. gpr) to the bound from the Builder.CreateICmpULT (8 in this context). Then the first (or any/every) use of the UsingOverflow area forces no more use of the UsingRegs area (for the involved va_list).
>> 
>> 
>> 
>> ===
>> Mark Millard
>> markmi at dsl-only.net
> 
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