Document revision date: 19 July 1999

The Alpha Architecture Reference Manual, Third Edition (AARM) describes strict rules for using interlocked memory instructions. The new 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 new 21264 processor. 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 DEC 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.

7.1 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 (named after the System Reference Manual, which defines the Alpha architecture) has been developed to analyze Alpha executables for noncompliant code sequences. The tool detects sequences that might fail, reports any errors, and displays the machine code of the failing sequence.

7.2 Using the Code Analysis Tool

The SRM_CHECK tool can be found in the following location on the OpenVMS Alpha Version 7.2 Operating System CD-ROM:

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. The -output qualifier can be used 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

The output from the tool can be used 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 had a nonzero base, it would be necessary 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.

7.3 Characteristics of Noncompliant Code

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 7.5 for information about compiler versions.

• Some versions of OpenVMS compilers introduce noncompliant code sequences during an optimization called "loop rotation." This problem can only be triggered 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, or DEC Pascal 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, the image should be recompiled with the appropriate compiler (see Section 7.5). After recompiling, the image should be analyzed again. If violations remain after recompiling, the source code must be examined to determine why the code scheduling violation exists. Modifications should then be made to the source code.

7.4 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. In this edition, Section 5.5, "Data Sharing", and Section 4.2.4, which describes the LDx_L instructions, detail 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 7.3.)

** 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.

7.5 Compiler Versions

This section contains information about versions of compilers that may generate noncompliant code sequences and the recommended versions to use when recompiling.

Table 7-1 contains information for OpenVMS compilers.

Table 7-1 OpenVMS Compilers

Old Version	Recommended Minimum Version
BLISS V1.1	BLISS V1.3
DEC C V5.x	DEC C V6.0
DEC C++ V5.x	DEC C++ V6.0
DEC Pascal V5.0-2	DEC Pascal V5.1-11
MACRO--32 V3.0	V3.1 for OpenVMS Version 7.1--2 V4.1 for OpenVMS Version 7.2
MACRO--64 V1.2	See below.

Current versions of the MACRO--64 assembler may 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.

7.6 Interlocked Memory Sequence Checking for the MACRO--32 Compiler

The MACRO--32 Compiler for OpenVMS Alpha Version 4.1 now performs additional code checking and displays warning messages for noncompliant code sequences. The following warning messages can display under the circumstances described:

BRNDIRLOC, branch directive ignored in locked memory sequence

BRNTRGLOC, branch target within locked memory sequence in routine 'routine_name'

MEMACCLOC, memory access within locked memory sequence in routine 'routine_name'

RETFOLLOC, RET/RSB follows LDx_L instruction

RTNCALLOC, routine call within locked memory sequence in routine 'routine_name'

STCMUSFOL, STx_C instruction must follow LDx_L instruction

Any MACRO--32 code on OpenVMS Alpha that invokes either the ALONONPAGED_INLINE or the LAL_REMOVE_FIRST macros from the SYS$LIBRARY:LIB.MLB macro library must be recompiled on OpenVMS Version 7.2 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 the new Alpha 21264 (EV6) processors.

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