64bit loop variable on 32bit may be optimized

[wzssyqa]

The Code like this

int x[128000];

void in(int k, int n) {
        for(unsigned long long i=0; i<128000; i++)
                x[i] = k;
}

If we build it for a 32bit system such as

./bin/clang --target=riscv32-linux-gnu -O3 -S n.c -emit-llvm

the 64bit one will be used, while in fact we can transform it to 32bit one, as it is less than 128000 always.

In fact gcc does this transform.

[frederick-vs-ja]

Godbolt link.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: YunQiang Su (wzssyqa)

The Code like this ``` int x[128000];

void in(int k, int n) {
for(unsigned long long i=0; i<128000; i++)
x[i] = k;
}

If we build it for a 32bit system such as

./bin/clang --target=riscv32-linux-gnu -O3 -S n.c -emit-llvm

the 64bit one will be used, while in fact we can transform it to 32bit one, as it is less than 128000 always.

In fact gcc does this transform.
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