Hi!
On the following testcase on powerpc64-linux (both -m32 and -m64) we emit
incorrect loop condition in the separate_ext loop.
For little endian, we do the addition on the least significant 3 limbs
(straight line code), 2 lowest limbs regular, the third on 7 bits and
then sign extension, and then a separate_ext loop
<bb 3> [local count: 1073741824]:
# _34 = PHI <3(2), _35(3)>
VIEW_CONVERT_EXPR<unsigned long[8]>(a)[_34] = _33;
_35 = _34 + 1;
if (_35 != 8)
goto <bb 3>; [0.05%]
else
goto <bb 4>; [99.95%]
to copy the sign extension into the remaining 5 limbs.
On big endian, we do the least significant 3 limbs similarly (of course,
the result is 512-bit, so with 56, 48, 40 instead of 0, 8, 16 on LE)
and then do the separate_ext loop:
<bb 3> [local count: 1073741824]:
# _35 = PHI <4(2), _36(3)>
VIEW_CONVERT_EXPR<unsigned long[8]>(a)[_35] = _34;
_36 = _35 + 18446744073709551615;
if (_36 != 0)
goto <bb 3>; [0.05%]
else
goto <bb 4>; [99.95%]
That is wrong, we start on limb 4 (i.e. offset 32) and go down until
1 (i.e. offset 8) rather than 0 (i.e. offset 0), because like in LE in this
case we compare idx_next (idx + increment, in BE case -1) instead of idx.
There was already code to handle specially BE rem != 0 case.
Note, the starting offset is
else if (i == (bo_shift != 0))
idx = create_loop (size_int (bo_idx
+ (bitint_big_endian
? total - 1 - start - i
: start + i)), &idx_next);
and we want to perform the same number of iterations on LE and BE,
just the LE offsets bo_idx + start + i + I for I going from 0 up until
bo_idx + start + i + I == bo_idx + end - 1 inclusive, while for BE
they should be bo_idx + total - 1 - start - i - I for I going from 0
up until bo_idx + total - 1 - start - i - I == bo_idx + total - 1 - end + 1
inclusive. So, the BE correction is to add total - 1 and subtract instead
of add the start + i up to end - 1 values.
So, this patch uses the exact same BE correction, with one exception.
If bo_idx + total - 1 - end + 1 is 0 (which is quite the common case,
when bo_idx is 0 (e.g. whenever it is not a bit-field store) and bo_shift
isn't non-zero either (again, e.g. no bit-field store) and rem == 0),
I think comparing idx != 0 instead of idx_next != (size_t) -1 is faster
(plus we'd in the evaluation need to care about types of all the addends,
do everything say in unsigned HOST_WIDE_INT and then map all ones in UHWI
to all ones in target size_t).
Bootstrapped/regtested on x86_64-linux, i686-linux (to make sure little
endian isn't broken), powerpc64-linux (--with-cpu=power8;
regtested there with both -m32/-m64 and additionally tested with
make -j192 -k check-gcc GCC_TEST_RUN_EXPENSIVE=1
RUNTESTFLAGS="GCC_TEST_RUN_EXPENSIVE=1 dg.exp='*bitint* pr112673.c
builtin-stdc-bit-*.c pr112566-2.c pr112511.c pr116588.c pr116003.c pr113693.c
pr113602.c flex-array-counted-by-7.c' dg-torture.exp='*bitint* pr116480-2.c
pr114312.c pr114121.c' dfp.exp=*bitint*
vect.exp='vect-early-break_99-pr113287.c' tree-ssa.exp=pr113735.c"
) and tested (non-bootstrapped) s390x-linux, again with the above
make check-gcc except with much lower -j. This fixes also
torture/bitint-16.c FAILs for -m32 on powerpc64-linux.
Ok for trunk?
2026-09-17 Jakub Jelinek <[email protected]>
Manjunath Matti <[email protected]>
PR middle-end/127378
* gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt): On
big endian, compare idx_next in separate_ext loop against
bo_idx + total - 1 - end, and if that wraps around, compare idx
against 0 instead.
* gcc.dg/bitint-145.c: New test.
--- a/gcc/gimple-lower-bitint.cc 2026-09-08 22:18:28.493021390 +0200
+++ b/gcc/gimple-lower-bitint.cc 2026-09-17 17:24:10.713167910 +0200
@@ -3271,15 +3271,18 @@ bitint_large_huge::lower_mergeable_stmt
bitint_big_endian
? size_int (-1) : size_one_node);
insert_before (g);
- if (bitint_big_endian && rem != 0)
- g = gimple_build_cond (NE_EXPR, idx,
- size_int (bo_idx + 1),
+ /* For big-endian, if bo_idx + total - 1 - end is all ones, then
+ compare idx (which is equal to idx_next + 1) against 0
+ instead. */
+ if (bitint_big_endian && bo_idx + total - end == 0)
+ g = gimple_build_cond (NE_EXPR, idx, size_zero_node,
NULL_TREE, NULL_TREE);
else
g = gimple_build_cond (NE_EXPR, idx_next,
size_int (bo_idx
+ (bitint_big_endian
- ? 0 : end)),
+ ? total - 1 - end
+ : end)),
NULL_TREE, NULL_TREE);
insert_before (g);
m_gsi = gsi_for_stmt (stmt);
--- a/gcc/testsuite/gcc.dg/bitint-145.c 2026-09-17 17:08:34.038519108 +0200
+++ b/gcc/testsuite/gcc.dg/bitint-145.c 2026-09-17 17:08:07.047875025 +0200
@@ -0,0 +1,35 @@
+/* PR middle-end/127378 */
+/* { dg-do run { target bitint } } */
+/* { dg-options "-std=c23 -O2" } */
+
+#if __BITINT_MAXWIDTH__ >= 768
+unsigned _BitInt(512) a;
+unsigned _BitInt(768) b = -1;
+
+__attribute__((noipa)) void
+foo (unsigned _BitInt(135) *p)
+{
+ a = (_BitInt(135)) (p[0] + p[1]);
+}
+
+__attribute__((noipa)) void
+bar (unsigned _BitInt(200) *p)
+{
+ b = p[0] + p[1];
+}
+#endif
+
+int
+main ()
+{
+#if __BITINT_MAXWIDTH__ >= 768
+ static unsigned _BitInt(135) p[2] = { -1, 0 };
+ static unsigned _BitInt(200) q[2] = { 1, 2 };
+ foo (p);
+ if (a != (unsigned _BitInt(512)) -1)
+ __builtin_abort ();
+ bar (q);
+ if (b != 3)
+ __builtin_abort ();
+#endif
+}
Jakub
