Batched GEMM: missing small-kernel guards, Fortran argument swaps, and empty-batch crash

Fermée

#6,121 ouverte le 12/10/2026 0 commentaires 0 réactions 0 personnes assignées Voir sur GitHub

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Évaluation

Difficulté: 4/5
Temps estimé: 3-5 jours
Accessibilité débutants: 32/100

Type d'issue: Bug
Clarté: Plutôt claire
Activité: Active
Stack technique: c, fortran
Domaine: backend

Piste de recherche

Four separate defects are listed across interface/gemm_batch.c, interface/gemm_batch_strided.c and kernel/generic/gemm_small_matrix_kernel_nn.c. Start by comparing the small-kernel dispatch in those interfaces with the alpha and permit checks in the ordinary GEMM path, then run the reproducer's check_a through check_d against a patched library. Done means all four checks return PASS, and the reproducer's exit status is 0.

Rédigé par le modèle d'indexation à partir du texte de l'issue.

Description

On develop 33746423, the batched GEMM interfaces differ from the corresponding ordinary GEMM operations in the four cases below. The original witnesses were reproduced on an x86-64 HASWELL build, natively on Apple M5, and under QEMU on arm64. The numerical cases use one matrix per batch; the zero-dimension case uses a two-member strided batch.

A. The alpha-zero guard is missing from batched small-matrix dispatch

The guard added to ordinary GEMM by #6083/#6085 is still missing from cblas_?gemm_batch and cblas_?gemm_batch_strided. With alpha=0, beta=1, C initialized to 1 and NaNs in A, both double-precision batch forms produce NaNs; the ordinary cblas_dgemm control leaves C unchanged.

gemm_batch.c:351 and gemm_batch_strided.c:364 select small kernels using only M·N·K <= 100³, without ordinary GEMM's alpha/permit checks. This shared dispatch affects the real and complex variants.

B. The Fortran interfaces swap operands or transpose flags

For column-major A = [1 3; 2 4] and B = [5 7; 6 8]:

Call	Ordinary GEMM control, in memory order	Batched result
`dgemm_batch_`, N,N	23 34 31 46	19 22 43 50 (B·A)
`dgemm_batch_strided_`, N,T	26 38 30 44	17 39 23 53 (Aᵀ·B)

The grouped Fortran branch exchanges TRANSA/TRANSB and passes b_array as A and a_array as B, without exchanging M/N. The strided Fortran branch keeps A/B in place but exchanges their transpose flags.

C. Zero-sized strided members leave uninitialized entries to dispatch

cblas_dgemm_batch_strided with M=0, N=K=2 and count=2 segfaults in the original witness. There are no C entries to update.

The zero-dimension branch skips initialization with continue, but the final dispatch still passes the original count. N=0 takes the same branch. The grouped interface instead maintains a compacted count.

D. SBGEMM batch reaches a small kernel that multiplies BF16 encodings

For BF16 inputs containing the same A and B, alpha=1 and beta=0, ordinary cblas_sbgemm returns 23 34 31 46. The strided batch returns approximately 5.41581e+08 5.44760e+08 5.43674e+08 5.46865e+08.

This shares the missing permit check in A. Ordinary SBGEMM's generic small-matrix permit is zero, but the batch interfaces bypass it and reach the generic multiplication with IFLOAT equal to uint16_t BF16 storage.

Reproducer

Linux, LP64 OpenBLAS (32-bit BLAS integers), with BF16 support enabled:

gcc -O2 -Wall -Wextra -Werror repro.c -ldl -lm -o repro
./repro /absolute/path/to/libopenblas.so

The checker below uses four-element matrix strides and checks each result against its exact expected value. Its exit status is 0 only when all four checks pass; mismatches or signals return 1, and missing symbols, failed controls or an incompatible integer configuration return 2. It sets one OpenBLAS thread.

The platform observations above come from the original witnesses. This revised checker adds explicit verdicts and conventional positive strides; it has not yet been rerun against the patched OpenBLAS library.

repro.c

/* Exact expected-result checks for batched GEMM on LP64 OpenBLAS builds.
 * Derived from the review packet's repro.c. A failed or missing control, a
 * fault, and an unchanged wrong output are failures, never evidence of a fix.
 * Usage: repro LIBRARY [CORETYPE]
 * All checks run with OPENBLAS_NUM_THREADS=1. This does not test parallel use.
 */
#define _GNU_SOURCE
#include <dlfcn.h>
#include <errno.h>
#include <math.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <unistd.h>

enum { Col = 102, N_ = 111, T_ = 112 };
enum { PASS = 0, FAIL = 1, ERROR = 2 };
typedef uint16_t bf16;
static void *lib;
static void *need(const char *name) {
    void *p = dlsym(lib, name);
    if (!p) { printf("MISSING %s\n", name); exit(ERROR); }
    return p;
}
typedef void (*dgemm_c)(int, int, int, int, int, int, double, const double *, int, const double *, int, double, double *, int);
typedef void (*dstr_c)(int, int, int, int, int, int, double, const double *, int, int, const double *, int, int, double, double *, int, int, int);
typedef void (*dbat_c)(int, const int *, const int *, const int *, const int *, const int *, const double *, const double **, const int *, const double **, const int *, const double *, double **, const int *, int, const int *);
typedef void (*dbat_f)(const char *, const char *, const int *, const int *, const int *, const double *, const double **, const int *, const double **, const int *, const double *, double **, const int *, const int *, const int *);
typedef void (*dstr_f)(const char *, const char *, const int *, const int *, const int *, const double *, const double *, const int *, const int *, const double *, const int *, const int *, const double *, double *, const int *, const int *, const int *);
typedef void (*sbgemm_c)(int, int, int, int, int, int, float, const bf16 *, int, const bf16 *, int, float, float *, int);
typedef void (*sbstr_c)(int, int, int, int, int, int, float, const bf16 *, int, int, const bf16 *, int, int, float, float *, int, int, int);

static void show(const char *label, const double *c) { printf("  %-34s C = %g %g %g %g\n", label, c[0], c[1], c[2], c[3]); }
static int same4(const double *x, const double *y) { return !memcmp(x, y, 4 * sizeof(double)); }
static int control_ok(const char *label, const double *c, const double *want) {
    if (same4(c, want)) return 1;
    printf("  CONTROL-WRONG %s: expected %g %g %g %g\n", label, want[0], want[1], want[2], want[3]);
    return 0;
}
static int check_a(void) {
    double a[4] = {NAN, NAN, NAN, NAN}, b[4] = {1, 1, 1, 1};
    double c0[4] = {1, 1, 1, 1}, c1[4] = {1, 1, 1, 1}, c2[4] = {1, 1, 1, 1};
    ((dgemm_c)need("cblas_dgemm"))(Col, N_, N_, 2, 2, 2, 0.0, a, 2, b, 2, 1.0, c0, 2);
    ((dstr_c)need("cblas_dgemm_batch_strided"))(Col, N_, N_, 2, 2, 2, 0.0, a, 2, 4, b, 2, 4, 1.0, c1, 2, 4, 1);
    int ta = N_, tb = N_, m = 2, n = 2, k = 2, ld = 2, gs = 1; double al = 0, be = 1;
    const double *ap[1] = {a}, *bp[1] = {b}; double *cp[1] = {c2};
    ((dbat_c)need("cblas_dgemm_batch"))(Col, &ta, &tb, &m, &n, &k, &al, ap, &ld, bp, &ld, &be, cp, &ld, 1, &gs);
    printf("A alpha=0, beta=1, A=NaN, C=1 (expected C unchanged)\n");
    show("cblas_dgemm (control)", c0); show("cblas_dgemm_batch_strided", c1); show("cblas_dgemm_batch", c2);
    const double ones[4] = {1, 1, 1, 1};
    if (!control_ok("cblas_dgemm", c0, ones)) return ERROR;
    return same4(c1, ones) && same4(c2, ones) ? PASS : FAIL;
}
static int check_b(void) {
    double a[4] = {1, 2, 3, 4}, b[4] = {5, 6, 7, 8}, c0[4] = {0}, c1[4] = {-1, -1, -1, -1}, c2[4] = {0}, c3[4] = {-1, -1, -1, -1};
    int m = 2, n = 2, k = 2, ld = 2, one = 1, s4 = 4; double al = 1, be = 0;
    dgemm_c g = (dgemm_c)need("cblas_dgemm");
    g(Col, N_, N_, 2, 2, 2, 1.0, a, 2, b, 2, 0.0, c0, 2);
    const double *ap[1] = {a}, *bp[1] = {b}; double *cp[1] = {c1};
    ((dbat_f)need("dgemm_batch_"))("N", "N", &m, &n, &k, &al, ap, &ld, bp, &ld, &be, cp, &ld, &one, &one);
    g(Col, N_, T_, 2, 2, 2, 1.0, a, 2, b, 2, 0.0, c2, 2);
    ((dstr_f)need("dgemm_batch_strided_"))("N", "T", &m, &n, &k, &al, a, &ld, &s4, b, &ld, &s4, &be, c3, &ld, &s4, &one);
    printf("B Fortran interfaces, A = [1 3; 2 4], B = [5 7; 6 8]\n");
    show("cblas_dgemm N,N (control)", c0); show("dgemm_batch_ N,N", c1);
    show("cblas_dgemm N,T (control)", c2); show("dgemm_batch_strided_ N,T", c3);
    const double nn[4] = {23, 34, 31, 46}, nt[4] = {26, 38, 30, 44};
    int ok0 = control_ok("cblas_dgemm N,N", c0, nn), ok2 = control_ok("cblas_dgemm N,T", c2, nt);
    if (!ok0 || !ok2) return ERROR;
    return same4(c1, nn) && same4(c3, nt) ? PASS : FAIL;
}
static int check_c(void) {
    double a[8] = {1, 1, 1, 1, 1, 1, 1, 1}, b[8] = {1, 1, 1, 1, 1, 1, 1, 1};
    double c0[8] = {5, 5, 5, 5, 5, 5, 5, 5};
    double c1[8] = {5, 5, 5, 5, 5, 5, 5, 5};
    printf("C cblas_dgemm_batch_strided M=0, N=K=2, count=2 (expected: return, no write)\n");
    ((dgemm_c)need("cblas_dgemm"))(Col, N_, N_, 0, 2, 2, 1.0, a, 2, b, 2, 1.0, c0, 2);
    for (int i = 0; i < 8; i++) if (c0[i] != 5) {
        printf("  CONTROL-WRONG cblas_dgemm M=0 at C[%d]\n", i); return ERROR;
    }
    ((dstr_c)need("cblas_dgemm_batch_strided"))(Col, N_, N_, 0, 2, 2, 1.0, a, 2, 4, b, 2, 4, 1.0, c1, 2, 4, 2);
    for (int i = 0; i < 8; i++) if (c1[i] != 5) {
        printf("  WRONG cblas_dgemm_batch_strided M=0: C[%d]=%g, want 5\n", i, c1[i]); return FAIL;
    }
    printf("  returned, all 8 C elements unchanged\n");
    return PASS;
}
static bf16 tobf(float f) { uint32_t u; memcpy(&u, &f, 4); return (bf16)(u >> 16); }
static int check_d(void) {
    bf16 a[4], b[4]; float c0[4] = {0}, c1[4] = {-1, -1, -1, -1};
    for (int i = 0; i < 4; i++) { a[i] = tobf((float)(i + 1)); b[i] = tobf((float)(i + 5)); }
    ((sbgemm_c)need("cblas_sbgemm"))(Col, N_, N_, 2, 2, 2, 1.0f, a, 2, b, 2, 0.0f, c0, 2);
    ((sbstr_c)need("cblas_sbgemm_batch_strided"))(Col, N_, N_, 2, 2, 2, 1.0f, a, 2, 4, b, 2, 4, 0.0f, c1, 2, 4, 1);
    printf("D SBGEMM, BF16 A = [1 3; 2 4], B = [5 7; 6 8], alpha=1, beta=0\n");
    printf("  %-34s C = %g %g %g %g\n", "cblas_sbgemm (control)", c0[0], c0[1], c0[2], c0[3]);
    printf("  %-34s C = %g %g %g %g\n", "cblas_sbgemm_batch_strided", c1[0], c1[1], c1[2], c1[3]);
    const float want[4] = {23, 34, 31, 46};
    if (memcmp(c0, want, sizeof want)) { printf("  CONTROL-WRONG cblas_sbgemm\n"); return ERROR; }
    return !memcmp(c1, want, sizeof want) ? PASS : FAIL;
}
int main(int argc, char **argv) {
    if (argc < 2) { fprintf(stderr, "usage: %s LIBRARY [CORETYPE]\n", argv[0]); return ERROR; }
    struct rlimit no_core = {0, 0}; setrlimit(RLIMIT_CORE, &no_core);
    setvbuf(stdout, NULL, _IOLBF, 0);
    if (argc > 2 && setenv("OPENBLAS_CORETYPE", argv[2], 1)) { perror("setenv"); return ERROR; }
    if (setenv("OPENBLAS_NUM_THREADS", "1", 1)) { perror("setenv"); return ERROR; }
    lib = dlopen(argv[1], RTLD_NOW | RTLD_LOCAL);
    if (!lib) { fprintf(stderr, "dlopen: %s\n", dlerror()); return ERROR; }
    const char *(*get_config)(void) = (const char *(*)(void))dlsym(lib, "openblas_get_config");
    if (get_config) {
        const char *config = get_config();
        printf("LIBRARY %s\nCONFIG %s\n", argv[1], config ? config : "unavailable");
        if (config && strstr(config, "USE64BITINT")) {
            fprintf(stderr, "ERROR: this reproducer requires LP64 OpenBLAS (32-bit BLAS integers).\n");
            return ERROR;
        }
    } else {
        printf("LIBRARY %s\nCONFIG unavailable; requires LP64 OpenBLAS (32-bit BLAS integers)\n", argv[1]);
    }
    int (*checks[4])(void) = {check_a, check_b, check_c, check_d};
    int passed = 0, failed = 0, errors = 0;
    for (int i = 0; i < 4; i++) {
        fflush(stdout);
        pid_t pid = fork();
        if (pid < 0) { perror("fork"); printf("CHECK %c ERROR fork\n", 'A' + i); errors++; continue; }
        if (pid == 0) { alarm(20); int rc = checks[i](); fflush(stdout); _exit(rc); }
        int st = 0; pid_t got;
        do { got = waitpid(pid, &st, 0); } while (got < 0 && errno == EINTR);
        if (got != pid) { perror("waitpid"); printf("CHECK %c ERROR waitpid\n", 'A' + i); errors++; }
        else if (WIFSIGNALED(st)) { printf("CHECK %c FAIL signal=%d\n", 'A' + i, WTERMSIG(st)); failed++; }
        else if (!WIFEXITED(st)) { printf("CHECK %c ERROR child status=%d\n", 'A' + i, st); errors++; }
        else if (WEXITSTATUS(st) == PASS) { printf("CHECK %c PASS\n", 'A' + i); passed++; }
        else if (WEXITSTATUS(st) == FAIL) { printf("CHECK %c FAIL\n", 'A' + i); failed++; }
        else { printf("CHECK %c ERROR child-exit=%d\n", 'A' + i, WEXITSTATUS(st)); errors++; }
    }
    printf("SUMMARY: %s passed=%d/4 failed=%d errors=%d core=%s threads=1\n",
           errors ? "ERROR" : failed ? "FAIL" : "PASS", passed, failed, errors, argc > 2 ? argv[2] : "dispatch");
    return errors ? ERROR : failed ? FAIL : PASS;
}

Candidate fix

The two-file diff below reuses ordinary GEMM's alpha/permit condition, corrects the Fortran argument mappings, and returns for zero-sized strided members. I am offering it as a candidate fix; I can prepare a PR with regression tests if useful.

batch-gemm.patch

diff --git a/interface/gemm_batch.c b/interface/gemm_batch.c
index 7a8789e..e36b861 100644
--- a/interface/gemm_batch.c
+++ b/interface/gemm_batch.c
@@ -298,19 +298,19 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE *  transa_array, enum CB
       group_ldb = ldb_array[i];
       group_ldc = ldc_array[i];
       
-      if (transb_array[i] == 'N')   group_transa = 0;
-      if (transb_array[i] == 'T')   group_transa = 1;
+      if (transa_array[i] == 'N')   group_transa = 0;
+      if (transa_array[i] == 'T')   group_transa = 1;
 #ifndef COMPLEX
-      if (transb_array[i] == 'C')   group_transa = 1;
+      if (transa_array[i] == 'C')   group_transa = 1;
 #else
-      if (transb_array[i] == 'C')   group_transa = 3;
+      if (transa_array[i] == 'C')   group_transa = 3;
 #endif
-      if (transa_array[i] == 'N')   group_transb = 0;
-      if (transa_array[i] == 'T')   group_transb = 1;
+      if (transb_array[i] == 'N')   group_transb = 0;
+      if (transb_array[i] == 'T')   group_transb = 1;
 #ifndef COMPLEX
-      if (transa_array[i] == 'C')   group_transb = 1;
+      if (transb_array[i] == 'C')   group_transb = 1;
 #else
-      if (transa_array[i] == 'C')   group_transb = 3;
+      if (transb_array[i] == 'C')   group_transb = 3;
 #endif
 #endif
 
@@ -348,7 +348,16 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE *  transa_array, enum CB
 #endif
 
 #ifdef SMALL_MATRIX_OPT
-    if (MNK <= 100.0*100.0*100.0){
+#if !defined(COMPLEX)
+    if (*(FLOAT *)(group_alpha) != ZERO &&
+        GEMM_SMALL_MATRIX_PERMIT(group_transa, group_transb, group_m, group_n, group_k,
+                                 *(FLOAT *)(group_alpha), *(FLOAT *)(group_beta))){
+#else
+    if (((FLOAT *)(group_alpha))[0] != ZERO &&
+        GEMM_SMALL_MATRIX_PERMIT(group_transa, group_transb, group_m, group_n, group_k,
+                                 ((FLOAT *)(group_alpha))[0], ((FLOAT *)(group_alpha))[1],
+                                 ((FLOAT *)(group_beta))[0], ((FLOAT *)(group_beta))[1])){
+#endif
       group_routine=NULL;
 #if !defined(COMPLEX)
       if(*(FLOAT *)(group_beta) == 0.0){
@@ -388,16 +397,15 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE *  transa_array, enum CB
       args_array[count].beta=group_beta;
 
 #if defined(CBLAS)
-      if (order == CblasColMajor) {      
-	args_array[count].a=(a_array[matrix_idx+j]);
-	args_array[count].b=(b_array[matrix_idx+j]);
-      }else if(order == CblasRowMajor){
-#endif
+      if (order == CblasRowMajor) {
 	args_array[count].a=(b_array[matrix_idx+j]);
 	args_array[count].b=(a_array[matrix_idx+j]);
-#if defined(CBLAS)
-      }
+      } else
 #endif
+      {
+	args_array[count].a=(a_array[matrix_idx+j]);
+	args_array[count].b=(b_array[matrix_idx+j]);
+      }
       
       args_array[count].c=(c_array[matrix_idx+j]);
       
diff --git a/interface/gemm_batch_strided.c b/interface/gemm_batch_strided.c
index f711b77..841933e 100644
--- a/interface/gemm_batch_strided.c
+++ b/interface/gemm_batch_strided.c
@@ -311,19 +311,19 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE transa, enum CBLAS_TRANS
       group_ldb = ldb;
       group_ldc = ldc;
       
-      if (tb == 'N')   group_transa = 0;
-      if (tb == 'T')   group_transa = 1;
+      if (ta == 'N')   group_transa = 0;
+      if (ta == 'T')   group_transa = 1;
 #ifndef COMPLEX
-      if (tb == 'C')   group_transa = 1;
+      if (ta == 'C')   group_transa = 1;
 #else
-      if (tb == 'C')   group_transa = 3;
+      if (ta == 'C')   group_transa = 3;
 #endif
-      if (ta == 'N')   group_transb = 0;
-      if (ta == 'T')   group_transb = 1;
+      if (tb == 'N')   group_transb = 0;
+      if (tb == 'T')   group_transb = 1;
 #ifndef COMPLEX
-      if (ta == 'C')   group_transb = 1;
+      if (tb == 'C')   group_transb = 1;
 #else
-      if (ta == 'C')   group_transb = 3;
+      if (tb == 'C')   group_transb = 3;
 #endif
 #endif
 
@@ -352,7 +352,8 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE transa, enum CBLAS_TRANS
       return;
     }
 
-    if (group_m == 0 || group_n == 0) continue;
+    /* Every member shares M and N: an empty member means an empty batch. */
+    if (group_m == 0 || group_n == 0) { free(args_array); return; }
 
     group_mode=mode;
 
@@ -361,7 +362,16 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE transa, enum CBLAS_TRANS
 #endif
 
 #ifdef SMALL_MATRIX_OPT
-    if (MNK <= 100.0*100.0*100.0){
+#if !defined(COMPLEX)
+    if (*(FLOAT *)(&alpha) != ZERO &&
+        GEMM_SMALL_MATRIX_PERMIT(group_transa, group_transb, group_m, group_n, group_k,
+                                 *(FLOAT *)(&alpha), *(FLOAT *)(&beta))){
+#else
+    if (((FLOAT *)(alpha))[0] != ZERO &&
+        GEMM_SMALL_MATRIX_PERMIT(group_transa, group_transb, group_m, group_n, group_k,
+                                 ((FLOAT *)(alpha))[0], ((FLOAT *)(alpha))[1],
+                                 ((FLOAT *)(beta))[0], ((FLOAT *)(beta))[1])){
+#endif
       group_routine=NULL;
 #if !defined(COMPLEX)
       if(beta == 0.0){

Found through AI-assisted code review and differential testing (Claude); the report and reproducer were also reviewed with GPT-6 Astra.

Langage dominant: C
Étoiles: 7.6k
Forks: 1.7k
Merge moyen: 1 j 11 h
PR mergées (30 j): 53

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area:gates bug good first issue

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InauguralSystems/EigenScript#1782 · Ouverte le 15/10/2026

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kubernetes filter: SIGSEGV with Use_Kubelet when kubelet /pods returns "items": null (uninitialized items_array in search_item_in_items)
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fluent/fluent-bit#12579 · Ouverte le 14/10/2026

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linux-v4l2: frame rate list truncates packed value to 32-bit int, so selected frame rate is ignored (driver falls back to max fps)
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obsproject/obs-studio#14036 · Ouverte le 14/10/2026

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swap-window -d with {current} target crashes server
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tmux/tmux#5742 · Ouverte le 14/10/2026

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Plus d'issues C

Batched GEMM: missing small-kernel guards, Fortran argument swaps, and empty-batch crash

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Évaluation

Piste de recherche

Description

A. The alpha-zero guard is missing from batched small-matrix dispatch

B. The Fortran interfaces swap operands or transpose flags

C. Zero-sized strided members leave uninitialized entries to dispatch

D. SBGEMM batch reaches a small kernel that multiplies BF16 encodings

Reproducer

Candidate fix

Préparer son environnement

Par où commencer

Autres issues de OpenMathLib/OpenBLAS

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