HP OpenVMS Systems Documentation

The Alpha Architecture Reference Manual, Third Edition (AARM) describes strict rules for using interlocked memory instructions. The Alpha 21264 (EV6) processor and all future Alpha processors are more stringent than their predecessors in their requirement that these rules be followed. As a result, code that has worked in the past, despite noncompliance, could fail when executed on systems featuring the 21264 processor and its successors. Occurrences of these noncompliant code sequences are believed to be rare. Note that the 21264 processor is not supported on versions prior to OpenVMS Alpha Version 7.1-2.

Noncompliant code can result in a loss of synchronization between processors when interprocessor locks are used, or can result in an infinite loop when an interlocked sequence always fails. Such behavior has occurred in some code sequences in programs compiled on old versions of the BLISS compiler, some versions of the MACRO--32 compiler and the MACRO--64 assembler, and in some HP C and C++ programs.

The affected code sequences use LDx_L/STx_C instructions, either directly in assembly language sources or in code generated by a compiler. Applications most likely to use interlocked instructions are complex, multithreaded applications or device drivers using highly optimized, hand-crafted locking and synchronization techniques.

A.1 Required Code Checks

OpenVMS recommends that code that will run on the 21264 processor be checked for these sequences. Particular attention should be paid to any code that does interprocess locking, multithreading, or interprocessor communication.

The SRM_CHECK tool has been developed to analyze the Alpha executables for noncompliant code sequences. The tool detects sequences that may fail, reports any errors, and displays the machine code of the failing sequence.

A.2 Using the Code Analysis Tool (SRM_CHECK)

The SRM_CHECK tool is located on the OpenVMS Alpha Version 7.3-2 Operating System CD:

SYS$SYSTEM:SRM_CHECK.EXE 

To run the SRM_CHECK tool, define it as a foreign command (or use the DCL$PATH mechanism) and invoke it with the name of the image to check. If a problem is found, the machine code is displayed and some image information is printed. The following example illustrates how to use the tool to analyze an image called myimage.exe:

$ define DCL$PATH []
$ srm_check myimage.exe

The tool supports wildcard searches. Use the following command line to initiate a wildcard search:

$ srm_check [*...]* -log

Use the -log qualifier to generate a list of images that have been checked. You can use the -output qualifier to write the output to a data file. For example, the following command directs output to a file named CHECK.DAT:

$ srm_check 'file' -output check.dat

You can use the output from the tool to find the module that generated the sequence by looking in the image's MAP file. The addresses shown correspond directly to the addresses that can be found in the MAP file.

The following example illustrates the output from using the analysis tool on an image named SYSTEM_SYNCHRONIZATION.EXE:

 
 ** Potential Alpha Architecture Violation(s) found in file... 
 ** Found an unexpected ldq at 00003618      
 0000360C   AD970130     ldq_l          R12, 0x130(R23) 
 00003610   4596000A     and            R12, R22, R10 
 00003614   F5400006     bne            R10, 00003630 
 00003618   A54B0000     ldq            R10, (R11) 
 Image Name:    SYSTEM_SYNCHRONIZATION 
 Image Ident:   X-3 
 Link Time:      5-NOV-1998 22:55:58.10 
 Build Ident:   X6P7-SSB-0000 
 Header Size:   584 
 Image Section: 0, vbn: 3, va: 0x0, flags: RESIDENT EXE (0x880) 
 

The MAP file for system_synchronization.exe contains the following:

EXEC$NONPAGED_CODE    00000000 0000B317 0000B318 (      45848.) 2 **  5 
SMPROUT               00000000 000047BB 000047BC (      18364.) 2 **  5 
SMPINITIAL            000047C0 000061E7 00001A28 (       6696.) 2 **  5 

The address 360C is in the SMPROUT module, which contains the addresses from 0-47BB. By looking at the machine code output from the module, you can locate the code and use the listing line number to identify the corresponding source code. If SMPROUT has a nonzero base, you need to subtract the base from the address (360C in this case) to find the relative address in the listing file.

Note that the tool reports potential violations in its output. Although SRM_CHECK can normally identify a code section in an image by the section's attributes, it is possible for OpenVMS images to contain data sections with those same attributes. As a result, SRM_CHECK may scan data as if it were code, and occasionally, a block of data may look like a noncompliant code sequence. This circumstance is rare and can be detected by examining the MAP and listing files.

A.3 Noncompliant Code Characteristics

The areas of noncompliance detected by the SRM_CHECK tool can be grouped into the following four categories. Most of these can be fixed by recompiling with new compilers. In rare cases, the source code may need to be modified. See Section A.5 for information about compiler versions.

• Some versions of OpenVMS compilers introduce noncompliant code sequences during an optimization called "loop rotation." This problem can be triggered only in C or C++ programs that use LDx_L/STx_C instructions in assembly language code that is embedded in the C/C++ source using the ASM function, or in assembly language written in MACRO--32 or MACRO--64. In some cases, a branch was introduced between the LDx_L and STx_C instructions.
  This can be addressed by recompiling.
• Some code compiled with very old BLISS, MACRO--32, DEC Pascal, or DEC COBOL compilers may contain noncompliant sequences. Early versions of these compilers contained a code scheduling bug where a load was incorrectly scheduled after a load_locked.
  This can be addressed by recompiling.
• In rare cases, the MACRO--32 compiler may generate a noncompliant code sequence for a BBSSI or BBCCI instruction where there are too few free registers.
  This can be addressed by recompiling.
• Errors may be generated by incorrectly coded MACRO--64 or MACRO--32 and incorrectly coded assembly language embedded in C or C++ source using the ASM function.
  This requires source code changes. The new MACRO--32 compiler flags noncompliant code at compile time.

If the SRM_CHECK tool finds a violation in an image, you should recompile the image with the appropriate compiler (see Section A.5). After recompiling, you should analyze the image again. If violations remain after recompiling, examine the source code to determine why the code scheduling violation exists. Then make the appropriate changes to the source code.

A.4 Coding Requirements

The Alpha Architecture Reference Manual describes how an atomic update of data between processors must be formed. The Third Edition, in particular, has much more information on this topic. This edition details the conventions of the interlocked memory sequence.

Exceptions to the following two requirements are the source of all known noncompliant code:

• There cannot be a memory operation (load or store) between the LDx_L (load locked) and STx_C (store conditional) instructions in an interlocked sequence.
• There cannot be a branch taken between an LDx_L and an STx_C instruction. Rather, execution must "fall through" from the LDx_L to the STx_C without taking a branch.
  Any branch whose target is between an LDx_L and matching STx_C creates a noncompliant sequence. For instance, any branch to "label" in the following example would result in noncompliant code, regardless of whether the branch instruction itself was within or outside of the sequence:

  LDx_L Rx, n(Ry) ... label: ... STx_C Rx, n(Ry)

Therefore, the SRM_CHECK tool looks for the following:

• Any memory operation (LDx/STx) between an LDx_L and an STx_C
• Any branch that has a destination between an LDx_L and an STx_C
• STx_C instructions that do not have a preceding LDx_L instruction
  This typically indicates that a backward branch is taken from an LDx_L to the STx_C Note that hardware device drivers that do device mailbox writes are an exception. These drivers use the STx_C to write the mailbox. This condition is found only on early Alpha systems and not on PCI-based systems.
• Excessive instructions between an LDx_L and an STxC
  The AARM recommends that no more than 40 instructions appear between an LDx_l and an STx_C. In theory, more than 40 instructions can cause hardware interrupts to keep the sequence from completing. However, there are no known occurrences of this.

To illustrate, the following are examples of code flagged by SRM_CHECK.

  ** Found an unexpected ldq at 0008291C 
  00082914   AC300000     ldq_l          R1, (R16) 
  00082918   2284FFEC     lda            R20, 0xFFEC(R4) 
  0008291C   A6A20038     ldq            R21, 0x38(R2) 

In the above example, an LDQ instruction was found after an LDQ_L before the matching STQ_C. The LDQ must be moved out of the sequence, either by recompiling or by source code changes. (See Section A.3.)

  ** Backward branch from 000405B0 to a STx_C sequence at 0004059C 
  00040598   C3E00003     br             R31, 000405A8 
  0004059C   47F20400     bis            R31, R18, R0 
  000405A0   B8100000     stl_c          R0, (R16) 
  000405A4   F4000003     bne            R0, 000405B4 
  000405A8   A8300000     ldl_l          R1, (R16) 
  000405AC   40310DA0     cmple          R1, R17, R0 
  000405B0   F41FFFFA     bne            R0, 0004059C 

In the above example, a branch was discovered between the LDL_L and STQ_C. In this case, there is no "fall through" path between the LDx_L and STx_C, which the architecture requires.

The following MACRO--32 source code demonstrates code where there is a "fall through" path, but this case is still noncompliant because of the potential branch and a memory reference in the lock sequence:

getlck: evax_ldql  r0, lockdata(r8)  ; Get the lock data 
        movl       index, r2         ; and the current index. 
        tstl       r0                ; If the lock is zero, 
        beql       is_clear          ; skip ahead to store. 
        movl       r3, r2            ; Else, set special index. 
is_clear: 
        incl       r0                ; Increment lock count 
        evax_stqc  r0, lockdata(r8)  ; and store it. 
        tstl       r0                ; Did store succeed? 
        beql       getlck            ; Retry if not. 

To correct this code, the memory access to read the value of INDEX must first be moved outside the LDQ_L/STQ_C sequence. Next, the branch between the LDQ_L and STQ_C, to the label IS_CLEAR, must be eliminated. In this case, it could be done using a CMOVEQ instruction. The CMOVxx instructions are frequently useful for eliminating branches around simple value moves. The following example shows the corrected code:

        movl       index, r2         ; Get the current index 
getlck: evax_ldql  r0, lockdata(r8)  ; and then the lock data. 
        evax_cmoveq r0, r3, r2       ; If zero, use special index. 
        incl       r0                ; Increment lock count 
        evax_stqc  r0, lockdata(r8)  ; and store it. 
        tstl       r0                ; Did write succeed? 
        beql       getlck            ; Retry if not. 

Table A-1 lists the versions of compilers that might generate noncompliant code sequences and the recommended minimum versions to use when you recompile.

Current versions of the MACRO--64 assembler might still encounter the loop rotation issue. However, MACRO--64 does not perform code optimization by default, and this problem occurs only when optimization is enabled. If SRM_CHECK indicates a noncompliant sequence in the MACRO--64 code, it should first be recompiled without optimization. If the sequence is still flagged when retested, the source code itself contains a noncompliant sequence that must be corrected.

Alpha computers with 21264 processors require strict adherence to the restrictions for interlocked memory sequences for the LDx_L and STx_C instructions described in the Alpha Architecture Reference Manual, Third Edition. To ensure that uses of interlocked memory instructions conform to the architectural guidelines, additional checking has been added to Version 3.1 of the MACRO--32 Compiler for OpenVMS Alpha.

The Alpha Architecture Reference Manual, Third Edition describes the rules for instruction use within interlocked memory sequences in Section 4.2.4. The MACRO--32 for OpenVMS Alpha Version 3.1 compiler observes these rules in the code it generates from MACRO--32 source code. However, the compiler provides EVAX_LQxL and EVAX_STxC built-ins, which allow these instructions to be written directly in source code.

The MACRO--32 Compiler for OpenVMS Alpha Version 4.1 now performs additional code checking and displays warning messages for noncompliant code sequences.

A.6 Recompiling Code with ALONONPAGED_INLINE or LAL_REMOVE_FIRST

Any MACRO--32 code on OpenVMS Alpha that invokes either the ALONONPAGED_INLINE or the LAL_REMOVE_FIRST macro from the SYS$LIBRARY:LIB.MLB macro library must be recompiled on OpenVMS Version 7.2 or higher to obtain a correct version of these macros. The change to these macros corrects a potential synchronization problem that is more likely to be encountered on newer processors, starting with Alpha 21264 (EV6).