[Bug target/121995] New: [RISCV] [Miscompile] GCC - riscv64 target, miscompiles with multiplication on unsigned char at -O3 as well as -O2

[skothadiya at whileone dot in via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121995

            Bug ID: 121995
           Summary: [RISCV] [Miscompile] GCC - riscv64 target, miscompiles
                    with multiplication on unsigned char at -O3 as well as
                    -O2
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: skothadiya at whileone dot in
  Target Milestone: ---

Created attachment 62415
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=62415&action=edit
Attached reduced testcase

GCC (riscv64 target) produces incorrect output for a simple C program when
compiled with -O2 or -O3.

Description:
The C code involves a loop with a multiplication operation on an unsigned char,
compiles correctly on x86_64 and produces the expected output. However, when
compiled for the riscv64 architecture, the resulting binary yields an incorrect
value. The expected output is 160, but the program returns 20. This
miscompilation occurs specifically with optimizations enabled at levels -O2 and
-O3.

A bug has been reported similar to this bug is #121985, However, this is
specifically affects an unsigned char variable.

Compiler flags:
-march=rv64gcv_zvl512b -flto -mrvv-vector-bits=zvl -mrvv-max-lmul=m8 -O3        

Reproduction Steps:
1. Compile the test case with GCC for riscv64 using the -O2 or -O3 flag.
2. Execute the compiled binary.
3. Observe the output.

COMMANDS:
/riscv-gnu-toolchain-build/bin/riscv64-unknown-linux-gnu-gcc
-march=rv64gcv_zvl512b -flto -mrvv-vector-bits=zvl -mrvv-max-lmul=m8 -O3 red.c
-o user-config.out -fsigned-char -fno-strict-aliasing -fwrapv
-Wno-unknown-warning-option -Werror -Wfatal-errors -Wall -Wformat
-Wno-int-in-bool-context -Wno-dangling-pointer
-Wno-compare-distinct-pointer-types -Wno-overflow -Wuninitialized
-Warray-bounds -Wreturn-type -Wno-unused-function -Wno-unused-variable
-Wno-unused-but-set-variable -Wno-unused-value -Wno-address -Wno-bool-compare
-Wno-pointer-sign -Wno-bool-operation -Wno-tautological-compare
-Wno-self-assign -Wno-implicit-const-int-float-conversion
-Wno-constant-conversion -Wno-unused-value
-Wno-tautological-constant-out-of-range-compare -Wno-constant-logical-operand
-Wno-parentheses-equality -Wno-pointer-sign

QEMU_CPU=rv64,vlen=512,rvv_ta_all_1s=true,rvv_ma_all_1s=true,v=true,vext_spec=v1.0,zve32f=true,zve64f=true
timeout --verbose -k 0.1 4
/home/skothadiya/CIFUZZRR/riscv-gnu-toolchain-build/bin/qemu-riscv64
user-config.out 1
20

  //Expected Output: 160
  //Actual Output: 20


-- testcase (red.c) --
unsigned short ak[2]={01,02};
unsigned char a = 5;

void b( unsigned short ak[]) {
    for (unsigned var=0; var<2; var++)
        for (char as=ak[var]; as<4; as+=1)
            a *= 2;
}
int printf(const char *, ...);

int main() {
    long long at;
    b(ak);
    at = (long long)a;
    printf("%llu\n", at);
}