[Bug target/121985] New: [RISCV] [Miscompile] GCC - riscv64 target, miscompiles at -O3 as well as -O2 on valid code

[skothadiya at whileone dot in via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121985

            Bug ID: 121985
           Summary: [RISCV] [Miscompile] GCC - riscv64 target, miscompiles
                    at -O3 as well as -O2 on valid code
           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 62408
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=62408&action=edit
Attached reduced testcase

GCC (riscv64 target) produces incorrect output for a simple C program when
compiled with -O2 or -O3. after 597b50abb0d2fc1f123a99e1b4beb89ad6259d9e.

Description:
The C code involves a loop with a multiplication operation on an unsigned short
integer, 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 9375, but the program returns
15. This miscompilation occurs specifically with optimizations enabled at
levels -O2 and -O3.

Compiler flags:
-march=rv64gcv_zvl512b -mrvv-vector-bits=zvl -mrvv-max-lmul=dynamic -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 -mrvv-vector-bits=zvl -mrvv-max-lmul=dynamic -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 /riscv-gnu-toolchain-build/bin/qemu-riscv64
user-config.out 1
15

  //Expected Output: 9375
  //Actual Output: 15


-- testcase (red.c) --

unsigned short a=3;
char f=1;
int printf(const char *, ...);

int main() {
        long long bx;
        for (char var=f; var<6; var++)
            a *= 5;

        printf("%hu\n", a);
}