> On 19 Aug 2026, at 14:06, Richard Biener <[email protected]> wrote:
>
> 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);
Will do, thanks.
>
>> + 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).
There's no gimple_build overload taking a function decl so that wouldn’t
compile, unless I’m not looking in the right place.
Thanks,
Kyrill
>
>> + 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)
