https://gcc.gnu.org/g:f5696e9b1113b0a8e464cabb8cc26c871b1d9289
commit r16-3220-gf5696e9b1113b0a8e464cabb8cc26c871b1d9289
Author: Vineet Gupta <vine...@rivosinc.com>
Date: Wed Aug 13 20:20:34 2025 -0700
RISC-V: fix __builtin_round clobbering FP exceptions flags [PR121534]
__builtin_round() fails to save/restore FP exception flags around the FP
compare insn which can potentially clobber the same.
Worth noting that the fflags restore bracketing is slightly different
than the glibc implementation. Both FLT and FCVT can potentially clobber
fflags. gcc generates below where even if branch is not taken and FCVT
is not executed, FLT still executed. Thus FSFLAGS is placed AFTER the
label 'L3'. glibc implementation FLT can't clobber due to early NaN check,
so FSFLAGS can be moved under the branch, before the label.
| convert_float_to_float_round
| ...
| frflags a5
| fabs.s fa5,fa0
| flt.s a4,fa5,fa4 <--- can clobber fflags
| beq a4,zero,.L3
| fcvt.w.s a4,fa0,rmm <--- also
| fcvt.s.w fa5,a4
| fsgnj.s fa0,fa5,fa0
| .L3:
| fsflags a5 <-- both code paths
Fixes: f652a35877e3 ("This is almost exclusively Jivan's work....")
PR target/121534
gcc/ChangeLog:
* config/riscv/riscv.md (round_pattern): save/restore fflags.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c: Adjust
scan pattern for additional instances of frflags/fsrflags.
Signed-off-by: Vineet Gupta <vine...@rivosinc.com>
Diff:
---
gcc/config/riscv/riscv.md | 12 ++++++++++++
.../gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c | 4 ++--
2 files changed, 14 insertions(+), 2 deletions(-)
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index af8adb096767..4718a75598a6 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -2313,12 +2313,16 @@
rtx abs_reg = gen_reg_rtx (<ANYF:MODE>mode);
rtx coeff_reg = gen_reg_rtx (<ANYF:MODE>mode);
rtx tmp_reg = gen_reg_rtx (<ANYF:MODE>mode);
+ rtx fflags = gen_reg_rtx (SImode);
riscv_emit_move (tmp_reg, operands[1]);
riscv_emit_move (coeff_reg,
riscv_vector::get_fp_rounding_coefficient
(<ANYF:MODE>mode));
emit_insn (gen_abs<ANYF:mode>2 (abs_reg, operands[1]));
+ /* fp compare can set invalid flag for NaN, so backup fflags. */
+ if (flag_trapping_math)
+ emit_insn (gen_riscv_frflags (fflags));
riscv_expand_conditional_branch (label, LT, abs_reg, coeff_reg);
emit_jump_insn (gen_jump (end_label));
@@ -2344,6 +2348,14 @@
emit_insn (gen_copysign<ANYF:mode>3 (tmp_reg, abs_reg, operands[1]));
emit_label (end_label);
+
+ /* Restore fflags, but after label. This is slightly different
+ than glibc implementation which only needs to restore under
+ the label, since it checks for NaN first, meaning following fp
+ compare can't raise fp exceptons and thus not clobber fflags. */
+ if (flag_trapping_math)
+ emit_insn (gen_riscv_fsflags (fflags));
+
riscv_emit_move (operands[0], tmp_reg);
}
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
index bb62ce2ef8a8..89af160112c9 100644
--- a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
@@ -54,5 +54,5 @@ DEF_OP_V (nearbyint, 512, double, __builtin_nearbyint)
/* { dg-final { scan-tree-dump-not "4096,4096" "optimized" } } */
/* { dg-final { scan-assembler-times
{vfcvt\.x\.f\.v\s+v[0-9]+,\s*v[0-9]+,\s*v0\.t} 30 } } */
/* { dg-final { scan-assembler-times
{vfcvt\.f\.x\.v\s+v[0-9]+,\s*v[0-9]+,\s*v0\.t} 30 } } */
-/* { dg-final { scan-assembler-times {frflags\s+[atx][0-9]+} 30 } } */
-/* { dg-final { scan-assembler-times {fsflags\s+[atx][0-9]+} 30 } } */
+/* { dg-final { scan-assembler-times {frflags\s+[atx][0-9]+} 32 } } */
+/* { dg-final { scan-assembler-times {fsflags\s+[atx][0-9]+} 32 } } */
