When one of the two input operands is 0, ADD and IOR are functionally
equivalent.
ADD is slightly preferred over IOR because ADD has a higher likelihood
of being implemented as a compressed instruction when compared to IOR.
C.ADD uses the CR format with any of the 32 RVI registers availble,
while C.OR uses the CA format with limit to just 8 of them.
Conditional select, if zero case:
rd = (rc == 0) ? rs1 : rs2
before patch:
czero.nez rd, rs1, rc
czero.eqz rtmp, rs2, rc
or rd, rd, rtmp
after patch:
czero.eqz rd, rs1, rc
czero.nez rtmp, rs2, rc
add rd, rd, rtmp
Same trick applies for the conditional select, if non-zero case:
rd = (rc != 0) ? rs1 : rs2
riscv-gnu-toolchain regression tests have been passed with no new failure.
---
gcc/config/riscv/riscv.cc | 2 +-
.../gcc.target/riscv/zicond-prefer-add-to-or.c | 16 ++++++++++++++++
2 files changed, 17 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index e5f00806bb9..93c736549c9 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -4709,7 +4709,7 @@ riscv_expand_conditional_move (rtx dest, rtx op, rtx
cons, rtx alt)
gen_rtx_IF_THEN_ELSE (mode, cond1,
CONST0_RTX (mode),
alt)));
- riscv_emit_binary (IOR, dest, reg1, reg2);
+ riscv_emit_binary (PLUS, dest, reg1, reg2);
return true;
}
}
diff --git a/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
b/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
new file mode 100644
index 00000000000..f3f7beb0b5e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zicond -mabi=lp64d -mbranch-cost=4" { target {
rv64 } } } */
+/* { dg-options "-march=rv32gc_zicond -mabi=ilp32f -mbranch-cost=4" { target {
rv32 } } } */
+/* { dg-skip-if "" { *-*-* } {"-O0" "-Og" "-Os" "-Oz"} } */
+
+long cond_select_if_zero(long a, long b, long c) {
+ return a == 0 ? c : b;
+}
+
+long cond_select_if_non_zero(long a, long b, long c) {
+ return a != 0 ? c : b;
+}
+
+/* { dg-final { scan-assembler-times {add\t} 2 } } */
+/* { dg-final { scan-assembler-not {or\t} } } */
+
--
2.17.1
