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[LoongArch] wrong code: select between a _BitInt(16) value and a sign-extended _BitInt(8) value is sign-extended from bit 7 #241868
Description
Live Reproducer: https://godbolt.org/z/TEfWGMefz
Compiling the following C code for loongarch64 with clang -O1 and running it prints the wrong result.
int printf(const char *, ...);
static volatile unsigned long long A = 128;
int main(void) {
_BitInt(16) a = A;
unsigned u = a;
signed char c = u;
long long r = 4ULL == u ? (_BitInt(8))~c : a;
printf("%llu\n", (unsigned long long)r);
return 0;
}clang command:
clang --target=loongarch64-linux-gnu -O1 repro.c && qemu-loongarch64 ./a.outOutput (wrong):
18446744073709551488
Expected (what -O0 prints, and what x86-64 prints):
128
u is 128, so the condition is false and r should be a, which is 128. The generated code instead sign-extends the selected value from bit 7:
ext.w.h $a1, $a0
bstrpick.d $a1, $a1, 31, 0
addi.d $a1, $a1, -4
sltui $a1, $a1, 1 # u == 4 -> 0
addi.d $a2, $zero, -1
maskeqz $a1, $a2, $a1 # mask = 0
xor $a0, $a0, $a1 # a ^ mask = 128
ext.w.b $a1, $a0 # r = (signed char)128 = -128 (wrong)Notes:
- Correct at -O0
clang version:
clang version 24.0.0git (https://github.com/llvm/llvm-project f5744e07d2dba7367ec68426fd6b6b07e2916b39)
Activity
llvmorg-github-actions commentedon Nov 12, 2026
@llvm/issue-subscribers-backend-loongarch
Author: Takuya Shizuka (hazohelet)
Details
Live Reproducer: https://godbolt.org/z/TEfWGMefzCompiling the following C code for
loongarch64withclang -O1and running it prints the wrong result.clang command:
Output (wrong):
Expected (what -O0 prints, and what x86-64 prints):
uis 128, so the condition is false andrshould bea, which is 128. The generated code instead sign-extends the selected value from bit 7:Notes:
clang version: