[glibc] x86: Fix unchecked AVX512-VBMI2 usage in strrchr-evex-base.S

[glibc] x86: Fix unchecked AVX512-VBMI2 usage in strrchr-evex-base.S

[Noah Goldstein]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b7f8b6b64b135e6dba5083d688675874d6809c91

commit b7f8b6b64b135e6dba5083d688675874d6809c91
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Wed Nov 1 16:42:07 2023 -0500

    x86: Fix unchecked AVX512-VBMI2 usage in strrchr-evex-base.S
    
    strrchr-evex-base used `vpcompress{b|d}` in the page cross logic but
    was missing the CPU_FEATURE checks for VBMI2 in the
    ifunc/ifunc-impl-list.
    
    The fix is either to add those checks or change the logic to not use
    `vpcompress{b|d}`. Choosing the latter here so that the strrchr-evex
    implementation is usable on SKX.
    
    New implementation is a bit slower, but this is in a cold path so its
    probably okay.

Diff:
---
 sysdeps/x86_64/multiarch/strrchr-evex-base.S | 75 +++++++++++++++++++---------
 1 file changed, 51 insertions(+), 24 deletions(-)

diff --git a/sysdeps/x86_64/multiarch/strrchr-evex-base.S b/sysdeps/x86_64/multiarch/strrchr-evex-base.S
index cd6a0a870a..da5bf1237a 100644
--- a/sysdeps/x86_64/multiarch/strrchr-evex-base.S
+++ b/sysdeps/x86_64/multiarch/strrchr-evex-base.S
@@ -35,18 +35,20 @@
 #  define CHAR_SIZE	4
 #  define VPCMP		vpcmpd
 #  define VPMIN		vpminud
-#  define VPCOMPRESS	vpcompressd
 #  define VPTESTN	vptestnmd
 #  define VPTEST	vptestmd
 #  define VPBROADCAST	vpbroadcastd
 #  define VPCMPEQ	vpcmpeqd
 
 # else
-#  define SHIFT_REG	VRDI
+#  if VEC_SIZE == 64
+#   define SHIFT_REG	VRCX
+#  else
+#   define SHIFT_REG	VRDI
+#  endif
 #  define CHAR_SIZE	1
 #  define VPCMP		vpcmpb
 #  define VPMIN		vpminub
-#  define VPCOMPRESS	vpcompressb
 #  define VPTESTN	vptestnmb
 #  define VPTEST	vptestmb
 #  define VPBROADCAST	vpbroadcastb
@@ -56,6 +58,12 @@
 #  define KORTEST_M	KORTEST
 # endif
 
+# if VEC_SIZE == 32 || (defined USE_AS_WCSRCHR)
+#  define SHIFT_R(cnt, val)	shrx cnt, val, val
+# else
+#  define SHIFT_R(cnt, val)	shr %cl, val
+# endif
+
 # define VMATCH		VMM(0)
 # define CHAR_PER_VEC	(VEC_SIZE / CHAR_SIZE)
 # define PAGE_SIZE	4096
@@ -71,7 +79,7 @@ ENTRY_P2ALIGN(STRRCHR, 6)
 	andl	$(PAGE_SIZE - 1), %eax
 	cmpl	$(PAGE_SIZE - VEC_SIZE), %eax
 	jg	L(cross_page_boundary)
-
+L(page_cross_continue):
 	VMOVU	(%rdi), %VMM(1)
 	/* k0 has a 1 for each zero CHAR in YMM1.  */
 	VPTESTN	%VMM(1), %VMM(1), %k0
@@ -79,7 +87,7 @@ ENTRY_P2ALIGN(STRRCHR, 6)
 	test	%VGPR(rsi), %VGPR(rsi)
 	jz	L(aligned_more)
 	/* fallthrough: zero CHAR in first VEC.  */
-L(page_cross_return):
+
 	/* K1 has a 1 for each search CHAR match in VEC(1).  */
 	VPCMPEQ	%VMATCH, %VMM(1), %k1
 	KMOV	%k1, %VGPR(rax)
@@ -167,7 +175,6 @@ L(first_vec_x1_return):
 
 	.p2align 4,, 12
 L(aligned_more):
-L(page_cross_continue):
 	/* Need to keep original pointer incase VEC(1) has last match.  */
 	movq	%rdi, %rsi
 	andq	$-VEC_SIZE, %rdi
@@ -340,34 +347,54 @@ L(return_new_match_ret):
 	leaq	(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %rax
 	ret
 
-	.p2align 4,, 4
 L(cross_page_boundary):
+	/* eax contains all the page offset bits of src (rdi). `xor rdi,
+	   rax` sets pointer will all page offset bits cleared so
+	   offset of (PAGE_SIZE - VEC_SIZE) will get last aligned VEC
+	   before page cross (guaranteed to be safe to read). Doing this
+	   as opposed to `movq %rdi, %rax; andq $-VEC_SIZE, %rax` saves
+	   a bit of code size.  */
 	xorq	%rdi, %rax
-	mov	$-1, %VRDX
-	VMOVU	(PAGE_SIZE - VEC_SIZE)(%rax), %VMM(6)
-	VPTESTN	%VMM(6), %VMM(6), %k0
+	VMOVU	(PAGE_SIZE - VEC_SIZE)(%rax), %VMM(1)
+	VPTESTN	%VMM(1), %VMM(1), %k0
 	KMOV	%k0, %VRSI
 
-# ifdef USE_AS_WCSRCHR
+	/* Shift out zero CHAR matches that are before the beginning of
+	   src (rdi).  */
+# if VEC_SIZE == 64 || (defined USE_AS_WCSRCHR)
 	movl	%edi, %ecx
-	and	$(VEC_SIZE - 1), %ecx
+# endif
+# ifdef USE_AS_WCSRCHR
+	andl	$(VEC_SIZE - 1), %ecx
 	shrl	$2, %ecx
 # endif
-	shlx	%SHIFT_REG, %VRDX, %VRDX
+	SHIFT_R	(%SHIFT_REG, %VRSI)
+# if VEC_SIZE == 32 || (defined USE_AS_WCSRCHR)
+	/* For strrchr-evex512 we use SHIFT_R as shr which will set zero
+	   flag.  */
+	test	%VRSI, %VRSI
+# endif
+	jz	L(page_cross_continue)
 
+	/* Found zero CHAR so need to test for search CHAR.  */
+	VPCMPEQ	%VMATCH, %VMM(1), %k1
+	KMOV	%k1, %VRAX
+	/* Shift out search CHAR matches that are before the beginning of
+	   src (rdi).  */
+	SHIFT_R	(%SHIFT_REG, %VRAX)
+	/* Check if any search CHAR match in range.  */
+	blsmsk	%VRSI, %VRSI
+	and	%VRSI, %VRAX
+	jz	L(ret2)
+	bsr	%VRAX, %VRAX
 # ifdef USE_AS_WCSRCHR
-	kmovw	%edx, %k1
+	leaq	(%rdi, %rax, CHAR_SIZE), %rax
 # else
-	KMOV	%VRDX, %k1
+	addq	%rdi, %rax
 # endif
-
-	VPCOMPRESS %VMM(6), %VMM(1){%k1}{z}
-	/* We could technically just jmp back after the vpcompress but
-	   it doesn't save any 16-byte blocks.  */
-	shrx	%SHIFT_REG, %VRSI, %VRSI
-	test	%VRSI, %VRSI
-	jnz	L(page_cross_return)
-	jmp	L(page_cross_continue)
-	/* 1-byte from cache line.  */
+L(ret2):
+	ret
+	/* 3 bytes from cache-line for evex.  */
+	/* 0 bytes from cache-line for evex512.  */
 END(STRRCHR)
 #endif