Date: Mon, 15 Feb 2016 00:52:15 -0800 From: Mark Millard <markmi@dsl-only.net> To: Roman Divacky <rdivacky@vlakno.cz>, Nathan Whitehorn <nwhitehorn@freebsd.org> Cc: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>, FreeBSD Toolchain <freebsd-toolchain@freebsd.org> Subject: Re: I've submitted 207175 for a clang 3.8.0 va_list handling problem for powerpc Message-ID: <D7D536A4-68B6-4506-BDFB-8C2C41E1C958@dsl-only.net> In-Reply-To: <70B405C4-E1AC-4F35-9786-051FDA2F8BE7@dsl-only.net> References: <F6846682-10F7-4D0D-A691-ED8D4366805C@dsl-only.net> <20160214192903.GA96697@vlakno.cz> <70B405C4-E1AC-4F35-9786-051FDA2F8BE7@dsl-only.net>
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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@dsl-only.net> wrote: > > On 2016-Feb-14, at 11:29 AM, Roman Divacky <rdivacky@vlakno.cz> wrote: >> >> Fwiw, the code to handle the vaarg is in >> tools/clang/lib/CodeGen/TargetInfo.cpp:PPC32_SVR4_ABIInfo::EmitVAArg() >> >> You can take a look to see whats wrong. >> >> On Sat, Feb 13, 2016 at 07:03:29PM -0800, Mark Millard wrote: >>> I've isolated another clang 3.8.0 TARGET_ARCH=powerpc SEGV problem that shows up for using clang 3.8.0 to buildworld/installworld for powerpc. >>> >>>> ls -l -n / >>> >>> gets a SEGV. As listed in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207175 ( and https://llvm.org/bugs/show_bug.cgi?id=26605 ) the following simplified program also gets the SEGV on powerpc: >>> >>>> #include <stdarg.h> // for va_list, va_start, va_arg, va_end >>>> #include <stdint.h> // for intmax_t >>>> >>>> intmax_t >>>> va_test (char *s, ...) >>>> { >>>> va_list vap; >>>> >>>> va_start(vap, s); >>>> >>>> char* t0 = va_arg(vap, char*); >>>> unsigned int o0 = va_arg(vap, unsigned int); >>>> int c0 = va_arg(vap, int); >>>> unsigned int u0 = va_arg(vap, unsigned int); >>>> int c1 = va_arg(vap, int); >>>> char * t1 = va_arg(vap, char*); >>>> >>>> intmax_t j0 = va_arg(vap, intmax_t); // This spans into overflow_arg_area. >>>> >>>> int c2 = va_arg(vap, int); // A copy was put in the >>>> // overflow_arg_area because of the >>>> // above. >>>> // But this tries to extract from the >>>> // last 4 bytes of the reg_save_area. >>>> // It does not increment the >>>> // overflow_arg_area position pointer >>>> // past the copy that is there. >>>> >>>> char * t2 = va_arg(vap, char*); // The lack of increment before makes >>>> // this extraction off by 4 bytes. >>>> >>>> char t2fc = *t2; // <<< This gets SEGV. t2 actually got what should be >>>> // the c2 value. >>>> >>>> intmax_t j1 = va_arg(vap, intmax_t); >>>> >>>> va_end(vap); >>>> >>>> return (intmax_t) ((s-t2)+(t0-t1)+o0+u0+j0+j1+c0+c1+c2+t2fc); >>>> // Avoid any optimize-away for lack of use. >>>> } >>>> >>>> int main(void) >>>> { >>>> char s[1025] = "test string for this"; >>>> >>>> char* t0 = s + 5; >>>> unsigned int o0 = 3; >>>> int c0 = 1; >>>> unsigned int u0 = 1; >>>> int c1 = 3; >>>> char * t1 = s + 12; >>>> intmax_t j0 = 314159265358979323; >>>> int c2 = 4; >>>> char * t2 = s + 16; >>>> intmax_t j1 = ~314159265358979323; >>>> >>>> intmax_t result = va_test(s,t0,o0,c0,u0,c1,t1,j0,c1,t2,j1); >>>> >>>> return (int) (result - (intmax_t) ((s-t2)+(t0-t1)+o0+u0+j0+j1+c0+c1+c2+*t2)); >>>> // Avoid any optimize-away for lack of use. >>>> } >>> >>> >>> >>> === >>> Mark Millard >>> markmi at dsl-only.net >>> >>> _______________________________________________ >>> freebsd-toolchain@freebsd.org mailing list >>> https://lists.freebsd.org/mailman/listinfo/freebsd-toolchain >>> To unsubscribe, send any mail to "freebsd-toolchain-unsubscribe@freebsd.org" > > 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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