Makefile + hash.h: remove PPC_SHA1 implementation

Remove the PPC_SHA1 implementation added in a6ef351 ([PATCH] PPC
assembly implementation of SHA1, 2005-04-22). When this was added
Apple consumer hardware used the PPC architecture, and the
implementation was intended to improve SHA-1 speed there.

Since it was added we've moved to using sha1collisiondetection by
default, and anyone wanting hard-rolled non-DC SHA-1 implementation
can use OpenSSL's via the OPENSSL_SHA1 knob.

The PPC_SHA1 originally originally targeted 32 bit PPC, and later the
64 bit PPC 970 (a.k.a. Apple PowerPC G5). See 926172c (block-sha1:
improve code on large-register-set machines, 2009-08-10) for a
reference about the performance on G5 (a comment in block-sha1/sha1.c
being removed here).

I can't get it to do anything but segfault on both the BE and LE POWER
machines in the GCC compile farm[1]. Anyone who's concerned about
performance on PPC these days is likely to be using the IBM POWER
processors.

There have been proposals to entirely remove non-sha1collisiondetection
implementations from the tree[2]. I think per [3] that would be a bit
overzealous. I.e. there are various set-ups git's speed is going to be
more important than the relatively implausible SHA-1 collision attack,
or where such attacks are entirely mitigated by other means (e.g. by
incoming objects being checked with DC_SHA1).

But that really doesn't apply to PPC_SHA1 in particular, which seems
to have outlived its usefulness.

As this gets rid of the only in-tree *.S assembly file we can remove
the small bits of logic from the Makefile needed to build objects
from *.S (as opposed to *.c)

The code being removed here was also throwing warnings with the
"-pedantic" flag, it could have been fixed as 544d93b (block-sha1:
remove use of obsolete x86 assembly, 2022-03-10) did for block-sha1/*,
but as noted above let's remove it instead.

1. https://cfarm.tetaneutral.net/machines/list/
   Tested on gcc{110,112,135,203}, a mixture of POWER [789] ppc64 and
   ppc64le. All segfault in anything needing object
   hashing (e.g. t/t1007-hash-object.sh) when compiled with
   PPC_SHA1=Y.
2. https://lore.kernel.org/git/[email protected]/
3. https://lore.kernel.org/git/[email protected]/

Acked-by: brian m. carlson" <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

INSTALL

@@ -135,8 +135,7 @@ Issues of note:
 
 	  By default, git uses OpenSSL for SHA1 but it will use its own
 	  library (inspired by Mozilla's) with either NO_OPENSSL or
-	  BLK_SHA1.  Also included is a version optimized for PowerPC
-	  (PPC_SHA1).
+	  BLK_SHA1.
 
 	- "libcurl" library is used for fetching and pushing
 	  repositories over http:// or https://, as well as by

Makefile

@@ -155,9 +155,6 @@ include shared.mak
 # Define BLK_SHA1 environment variable to make use of the bundled
 # optimized C SHA1 routine.
 #
-# Define PPC_SHA1 environment variable when running make to make use of
-# a bundled SHA1 routine optimized for PowerPC.
-#
 # Define DC_SHA1 to unconditionally enable the collision-detecting sha1
 # algorithm. This is slower, but may detect attempted collision attacks.
 # Takes priority over other *_SHA1 knobs.
@@ -1802,6 +1799,10 @@ ifdef APPLE_COMMON_CRYPTO
 	SHA1_MAX_BLOCK_SIZE = 1024L*1024L*1024L
 endif
 
+ifdef PPC_SHA1
+$(error the PPC_SHA1 flag has been removed along with the PowerPC-specific SHA-1 implementation.)
+endif
+
 ifdef OPENSSL_SHA1
 	EXTLIBS += $(LIB_4_CRYPTO)
 	BASIC_CFLAGS += -DSHA1_OPENSSL
@@ -1810,10 +1811,6 @@ ifdef BLK_SHA1
 	LIB_OBJS += block-sha1/sha1.o
 	BASIC_CFLAGS += -DSHA1_BLK
 else
-ifdef PPC_SHA1
-	LIB_OBJS += ppc/sha1.o ppc/sha1ppc.o
-	BASIC_CFLAGS += -DSHA1_PPC
-else
 ifdef APPLE_COMMON_CRYPTO
 	COMPAT_CFLAGS += -DCOMMON_DIGEST_FOR_OPENSSL
 	BASIC_CFLAGS += -DSHA1_APPLE
@@ -1847,7 +1844,6 @@ endif
 endif
 endif
 endif
-endif
 
 ifdef OPENSSL_SHA256
 	EXTLIBS += $(LIB_4_CRYPTO)
@@ -2594,14 +2590,10 @@ missing_compdb_dir =
 compdb_args =
 endif
 
-ASM_SRC := $(wildcard $(OBJECTS:o=S))
-ASM_OBJ := $(ASM_SRC:S=o)
-C_OBJ := $(filter-out $(ASM_OBJ),$(OBJECTS))
+C_OBJ := $(OBJECTS)
 
 $(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
-$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
-	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
 
 %.s: %.c GIT-CFLAGS FORCE
 	$(QUIET_CC)$(CC) -o $@ -S $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<

block-sha1/sha1.c

@@ -28,10 +28,6 @@
  * try to do the silly "optimize away loads" part because it won't
  * see what the value will be).
  *
- * Ben Herrenschmidt reports that on PPC, the C version comes close
- * to the optimized asm with this (ie on PPC you don't want that
- * 'volatile', since there are lots of registers).
- *
  * On ARM we get the best code generation by forcing a full memory barrier
  * between each SHA_ROUND, otherwise gcc happily get wild with spilling and
  * the stack frame size simply explode and performance goes down the drain.

configure.ac

@@ -237,9 +237,6 @@ AC_MSG_NOTICE([CHECKS for site configuration])
 # tests.  These tests take up a significant amount of the total test time
 # but are not needed unless you plan to talk to SVN repos.
 #
-# Define PPC_SHA1 environment variable when running make to make use of
-# a bundled SHA1 routine optimized for PowerPC.
-#
 # Define NO_OPENSSL environment variable if you do not have OpenSSL.
 #
 # Define OPENSSLDIR=/foo/bar if your openssl header and library files are in

hash.h

@@ -4,9 +4,7 @@
 #include "git-compat-util.h"
 #include "repository.h"
 
-#if defined(SHA1_PPC)
-#include "ppc/sha1.h"
-#elif defined(SHA1_APPLE)
+#if defined(SHA1_APPLE)
 #include <CommonCrypto/CommonDigest.h>
 #elif defined(SHA1_OPENSSL)
 #include <openssl/sha.h>
@@ -32,7 +30,7 @@
  * platform's underlying implementation of SHA-1; could be OpenSSL,
  * blk_SHA, Apple CommonCrypto, etc...  Note that the relevant
  * SHA-1 header may have already defined platform_SHA_CTX for our
- * own implementations like block-sha1 and ppc-sha1, so we list
+ * own implementations like block-sha1, so we list
  * the default for OpenSSL compatible SHA-1 implementations here.
  */
 #define platform_SHA_CTX	SHA_CTX