This patch implement new pattern for match.pd that recognize the
sign-extension idiom (x & low_bits) - (x & sign_bit) and tests that covers it.
Pattern is based on conversation from bug report.
TRUNK riscv output:
sext8_hd:
andi a5,a0,127
andi a0,a0,128
sub a0,a5,a0
ret
sext16_hd:
slli a5,a0,49
li a4,32768
srli a5,a5,49
and a0,a0,a4
sub a0,a5,a0
ret
NEW riscv output:
sext8_hd:
slliw a0,a0,24
sraiw a0,a0,24
ret
sext16_hd:
slliw a0,a0,16
sraiw a0,a0,16
ret
TRUNK x86 output:
"sext8_hd":
mov rax, rdi
and edi, 128
and eax, 127
sub rax, rdi
ret
"sext16_hd":
mov rax, rdi
and edi, 32768
and eax, 32767
sub rax, rdi
ret
new x86 output:
sext8_hd:
movsbq %dil, %rax
ret
sext16_hd:
movswq %di, %rax
ret
2026-06-15 Milan Tripkovic <[email protected]>
PR target/125405
gcc/ChangeLog:
* match.pd: New pattern added
gcc/testsuite/ChangeLog:
* gcc.dg/pr125405-bitint.c: New test.
* gcc.dg/tree-ssa/pr125405.c: New test.
* gcc.target/riscv/pr125405.c: New test.
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---
gcc/match.pd | 17 +++++++++++
gcc/testsuite/gcc.dg/pr125405-bitint.c | 36 +++++++++++++++++++++++
gcc/testsuite/gcc.dg/tree-ssa/pr125405.c | 24 +++++++++++++++
gcc/testsuite/gcc.target/riscv/pr125405.c | 22 ++++++++++++++
4 files changed, 99 insertions(+)
create mode 100644 gcc/testsuite/gcc.dg/pr125405-bitint.c
create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr125405.c
create mode 100644 gcc/testsuite/gcc.target/riscv/pr125405.c
diff --git a/gcc/match.pd b/gcc/match.pd
index e0d7ef80e1..32d803f362 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -4927,6 +4927,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(if (stype && (width == 1 || type_has_mode_precision_p (stype)))
(convert (convert:stype @0))))))))
+/* PR125405: (x & ((1 << (N - 1)) - 1)) - (x & (1 << (N - 1)))
+ -> sign_extend<N> (x). */
+(simplify
+ (minus (bit_and @0 INTEGER_CST@1) (bit_and @0 integer_pow2p@2))
+ (if (INTEGRAL_TYPE_P (type)
+ && TYPE_PRECISION (type) <= MAX_FIXED_MODE_SIZE
+ && wi::to_wide (@1) + 1 == wi::to_wide (@2))
+ (with {
+ int width = wi::exact_log2 (wi::to_wide (@2)) + 1;
+ tree ntype = NULL_TREE;
+
+ if (width > 0 && width < TYPE_PRECISION (type))
+ ntype = build_nonstandard_integer_type (width, SIGNED);
+ }
+ (if (ntype && type_has_mode_precision_p (ntype))
+ (convert (convert:ntype @0))))))
+
/* Optimize x >> x into 0 */
(simplify
(rshift @0 @0)
diff --git a/gcc/testsuite/gcc.dg/pr125405-bitint.c
b/gcc/testsuite/gcc.dg/pr125405-bitint.c
new file mode 100644
index 0000000000..857dc0f7b3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr125405-bitint.c
@@ -0,0 +1,36 @@
+/* PR tree-optimization/125405 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned _BitInt(32)
+sext8_bitint32 (unsigned _BitInt(32) x)
+{
+ return (x & (unsigned _BitInt(32)) 0x7f)
+ - (x & (unsigned _BitInt(32)) 0x80);
+}
+
+unsigned _BitInt(32)
+sext16_bitint32 (unsigned _BitInt(32) x)
+{
+ return (x & (unsigned _BitInt(32)) 0x7fff)
+ - (x & (unsigned _BitInt(32)) 0x8000);
+}
+
+unsigned _BitInt(32)
+no_sext_bad_masks (unsigned _BitInt(32) x)
+{
+ return (x & (unsigned _BitInt(32)) 0x7e)
+ - (x & (unsigned _BitInt(32)) 0x80);
+}
+
+unsigned _BitInt(32)
+no_sext_bad_pair (unsigned _BitInt(32) x)
+{
+ return (x & (unsigned _BitInt(32)) 0xff)
+ - (x & (unsigned _BitInt(32)) 0x80);
+}
+
+/* { dg-final { scan-tree-dump-times "\\(signed char\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\\(signed short\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump " & 126" "optimized" } } */
+/* { dg-final { scan-tree-dump " & 255" "optimized" } } */
\ No newline at end of file
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr125405.c
b/gcc/testsuite/gcc.dg/tree-ssa/pr125405.c
new file mode 100644
index 0000000000..175c6b7af6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr125405.c
@@ -0,0 +1,24 @@
+/* PR tree-optimization/125405 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+typedef unsigned long xlen_t;
+
+xlen_t
+sext8_hd (xlen_t x)
+{
+ return (x & 0x7fUL) - (x & 0x80UL);
+}
+
+xlen_t
+sext16_hd (xlen_t x)
+{
+ return (x & 0x7fffUL) - (x & 0x8000UL);
+}
+
+/* { dg-final { scan-tree-dump-times "\\(signed char\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\\(signed short\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-not " & 127" "optimized" } } */
+/* { dg-final { scan-tree-dump-not " & 128" "optimized" } } */
+/* { dg-final { scan-tree-dump-not " & 32767" "optimized" } } */
+/* { dg-final { scan-tree-dump-not " & 32768" "optimized" } } */
\ No newline at end of file
diff --git a/gcc/testsuite/gcc.target/riscv/pr125405.c
b/gcc/testsuite/gcc.target/riscv/pr125405.c
new file mode 100644
index 0000000000..2aacad89b6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr125405.c
@@ -0,0 +1,22 @@
+/* PR tree-optimization/125405 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=rv64gc -mabi=lp64d" } */
+
+typedef unsigned long xlen_t;
+
+xlen_t
+sext8_hd (xlen_t x)
+{
+ return (x & 0x7fUL) - (x & 0x80UL);
+}
+
+xlen_t
+sext16_hd (xlen_t x)
+{
+ return (x & 0x7fffUL) - (x & 0x8000UL);
+}
+
+/* { dg-final { scan-assembler-times "\\mslliw\\M" 2 } } */
+/* { dg-final { scan-assembler-times "\\msraiw\\M" 2 } } */
+/* { dg-final { scan-assembler-not "\\mandi\\M" } } */
+/* { dg-final { scan-assembler-not "\\msub\\M" } } */
\ No newline at end of file
--
2.34.1
