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)
>
