Re: [PATCH] gimple-fold: fold fwrite of a single byte to fputc

On Wed, Aug 19, 2026 at 1:03 PM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> fwrite (p, 1, 1, f) transfers one byte and, when nothing looks at its result,
> writes exactly what fputc (*p, f) writes.  fputc reaches the stream directly
> while fwrite goes through the generic buffered-write path, so the fold is
> worth making.  GCC already treats the two as interchangeable in the other
> direction, since gimple_fold_builtin_fputs turns a one-character fputs into
> fputc and a longer one into fwrite.  There was simply no folder for fwrite.
>
> The unlocked entry points fold the same way, into fputc_unlocked, on the
> assumption gimple_fold_builtin_fputs already makes: a program that calls one
> unlocked stdio function has the others available too.
>
> jq emits its output one character at a time through a helper that ends in
> fwrite (&c, 1, 1, f), which is the shape this fold is aimed at:
>
>     before                              after
>         stp     x29, x30, [sp, -32]!    and     w0, w0, 255
>         mov     w2, 1                   b       fputc
>         mov     x29, sp
>         mov     x3, x1
>         uxtw    x1, w2
>         strb    w0, [sp, 31]
>         add     x0, sp, 31
>         bl      fwrite
>         ldp     x29, x30, [sp], 32
>         ret
>
> Measured with jq 1.8.2 over a 57 MB NDJSON corpus on Grace with
> -mcpu=grace -O3 gets these speedups.
>
>     jq -c .                             -30.30% instructions, -44.80% cycles
>     jq -c -f <a map/select filter>       -2.92% instructions,  -5.67% cycles
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
>         * gimple-fold.cc (gimple_fold_builtin_fwrite): New function.
>         (gimple_fold_builtin): Call it for BUILT_IN_FWRITE and
>         BUILT_IN_FWRITE_UNLOCKED.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/fwrite-1.c: New test.
>         * gcc.dg/fwrite-2.c: New test.
>         * gcc.dg/fwrite-3.c: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>  gcc/gimple-fold.cc              | 47 +++++++++++++++++++++++++++++++++
>  gcc/testsuite/gcc.dg/fwrite-1.c | 15 +++++++++++
>  gcc/testsuite/gcc.dg/fwrite-2.c | 32 ++++++++++++++++++++++
>  gcc/testsuite/gcc.dg/fwrite-3.c | 25 ++++++++++++++++++
>  4 files changed, 119 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/fwrite-1.c
>  create mode 100644 gcc/testsuite/gcc.dg/fwrite-2.c
>  create mode 100644 gcc/testsuite/gcc.dg/fwrite-3.c
>
> diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
> index 24041466108..583afe119e7 100644
> --- a/gcc/gimple-fold.cc
> +++ b/gcc/gimple-fold.cc
> @@ -3099,6 +3099,49 @@ gimple_fold_builtin_fputs (gimple_stmt_iterator *gsi,
>      }
>  }
>
> +/* Fold a call to fwrite (PTR, SIZE, N, STREAM) at *GSI.  UNLOCKED says whether
> +   the callee is fwrite_unlocked rather than fwrite.  A call that transfers a
> +   single byte and whose result is nobody's business writes the same byte as
> +   fputc (*PTR, STREAM), which reaches the stream without going through the
> +   generic buffered-write path.  Return true if the call was folded.  */
> +
> +static bool
> +gimple_fold_builtin_fwrite (gimple_stmt_iterator *gsi, bool unlocked)
> +{
> +  gimple *stmt = gsi_stmt (*gsi);
> +
> +  /* fwrite reports the number of items transferred and fputc the character
> +     written, so only fold when nothing looks at the result.  */
> +  if (gimple_call_lhs (stmt))
> +    return false;
> +
> +  /* fwrite transfers SIZE * N bytes, so writing a single byte needs both
> +     counts to be one: no other pair of non-negative values multiplies to
> +     one.  */
> +  if (!integer_onep (gimple_call_arg (stmt, 1))
> +      || !integer_onep (gimple_call_arg (stmt, 2)))
> +    return false;
> +
> +  /* If we're using an unlocked function, assume the other unlocked
> +     functions exist explicitly.  */
> +  tree const fn_fputc = (unlocked
> +                        ? builtin_decl_explicit (BUILT_IN_FPUTC_UNLOCKED)
> +                        : builtin_decl_implicit (BUILT_IN_FPUTC));
> +  if (!fn_fputc || (!gimple_vdef (stmt) && gimple_in_ssa_p (cfun)))
> +    return false;
> +
> +  location_t loc = gimple_location (stmt);
> +  gimple_seq stmts = NULL;
> +  tree byte = gimple_load_first_char (loc, gimple_call_arg (stmt, 0), &stmts);
> +  tree c = make_ssa_name (integer_type_node);
> +  gimple_seq_add_stmt_without_update (&stmts,
> +                                     gimple_build_assign (c, NOP_EXPR, byte));

   tree c = gimple_convert (&stmts, integer_type_node, byte);

> +  gimple_seq_add_stmt_without_update
> +    (&stmts, gimple_build_call (fn_fputc, 2, c, gimple_call_arg (stmt, 3)));

  gimple_build (&stmts, fn_putc, void_type_node, c, gimple_call_arg (stmt, 3));

might work here (unsure about calls with no LHS).

> +  gsi_replace_with_seq_vops (gsi, stmts);
> +  return true;
> +}
> +
>  /* Fold a call to the __mem{cpy,pcpy,move,set}_chk builtin.
>     DEST, SRC, LEN, and SIZE are the arguments to the call.
>     IGNORE is true, if return value can be ignored.  FCODE is the BUILT_IN_*
> @@ -5583,6 +5626,10 @@ gimple_fold_builtin (gimple_stmt_iterator *gsi)
>      case BUILT_IN_FPUTS_UNLOCKED:
>        return gimple_fold_builtin_fputs (gsi, gimple_call_arg (stmt, 0),
>                                         gimple_call_arg (stmt, 1), true);
> +    case BUILT_IN_FWRITE:
> +      return gimple_fold_builtin_fwrite (gsi, false);
> +    case BUILT_IN_FWRITE_UNLOCKED:
> +      return gimple_fold_builtin_fwrite (gsi, true);
>      case BUILT_IN_MEMCPY_CHK:
>      case BUILT_IN_MEMPCPY_CHK:
>      case BUILT_IN_MEMMOVE_CHK:
> diff --git a/gcc/testsuite/gcc.dg/fwrite-1.c b/gcc/testsuite/gcc.dg/fwrite-1.c
> new file mode 100644
> index 00000000000..5e3a5043e9a
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/fwrite-1.c
> @@ -0,0 +1,15 @@
> +/* fwrite of a single byte whose result is unused writes the same byte as
> +   fputc, so it should be folded.  */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -Wno-unused-result -fdump-tree-optimized" } */
> +
> +#include <stdio.h>
> +
> +void one (FILE *f, const char *p) { fwrite (p, 1, 1, f); }
> +size_t used (FILE *f, const char *p) { return fwrite (p, 1, 1, f); }
> +void two_items (FILE *f, const char *p) { fwrite (p, 1, 2, f); }
> +void two_bytes (FILE *f, const char *p) { fwrite (p, 2, 1, f); }
> +void unknown (FILE *f, const char *p, size_t n) { fwrite (p, 1, n, f); }
> +
> +/* { dg-final { scan-tree-dump-times "fputc" 1 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "fwrite" 4 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/fwrite-2.c b/gcc/testsuite/gcc.dg/fwrite-2.c
> new file mode 100644
> index 00000000000..7565f52def8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/fwrite-2.c
> @@ -0,0 +1,32 @@
> +/* Check that folding fwrite of a single byte to fputc keeps the output the
> +   same, including the bytes that are not folded, and that a side effect in the
> +   stream argument is still evaluated exactly once.  */
> +/* { dg-do run } */
> +/* { dg-options "-O2 -Wno-unused-result" } */
> +
> +#include <stdio.h>
> +
> +extern void abort (void);
> +
> +const char s[] = "abcd";
> +
> +int
> +main (void)
> +{
> +  FILE *streams[] = { stdout, NULL }, **p = streams;
> +
> +  fwrite (s, 1, 1, stdout);
> +  fwrite (s + 1, 1, 1, stdout);
> +  fwrite (s + 2, 1, 2, stdout);
> +  fwrite (s, 2, 1, stdout);
> +
> +  /* The folded call must still advance P exactly once.  */
> +  fwrite (s, 1, 1, *p++);
> +  if (p != streams + 1 || *p != NULL)
> +    abort ();
> +
> +  fflush (stdout);
> +  return 0;
> +}
> +
> +/* { dg-output "abcdaba" } */
> diff --git a/gcc/testsuite/gcc.dg/fwrite-3.c b/gcc/testsuite/gcc.dg/fwrite-3.c
> new file mode 100644
> index 00000000000..6ee0f58cb59
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/fwrite-3.c
> @@ -0,0 +1,25 @@
> +/* fwrite_unlocked of a single byte whose result is unused writes the same byte
> +   as fputc_unlocked, so it is folded the same way as the locked form, and into
> +   the unlocked entry point rather than the locked one.  */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -Wno-unused-result -fdump-tree-optimized" } */
> +
> +#include <stdio.h>
> +
> +/* Declared here rather than through _GNU_SOURCE so that the test does not
> +   depend on the host header exposing the unlocked entry points.  */
> +extern int (fputc_unlocked) (int, FILE *);
> +extern size_t (fwrite_unlocked) (const void *, size_t, size_t, FILE *);
> +
> +void one (FILE *f, const char *p) { fwrite_unlocked (p, 1, 1, f); }
> +size_t used (FILE *f, const char *p) { return fwrite_unlocked (p, 1, 1, f); }
> +void two_items (FILE *f, const char *p) { fwrite_unlocked (p, 1, 2, f); }
> +void two_bytes (FILE *f, const char *p) { fwrite_unlocked (p, 2, 1, f); }
> +void unknown (FILE *f, const char *p, size_t n) { fwrite_unlocked (p, 1, n, f); }
> +
> +/* { dg-final { scan-tree-dump-times "fputc_unlocked" 1 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "fwrite_unlocked" 4 "optimized" } } */
> +/* The unlocked form must not be folded into the locked fputc.  A call with no
> +   result is dumped as "  fputc (...", which the leading space matches without
> +   also matching fputc_unlocked.  */
> +/* { dg-final { scan-tree-dump-not " fputc \\(" "optimized" } } */
> --
> 2.50.1 (Apple Git-155)
>