compiler-rt: alu: add saturated shift left for i8 #134

[wzmuda]

Summary

Implement the following polyfills:

• __llvm_ushl_sat_i8_i8
• __llvm_sshl_sat_i8_i8

Also do some minor refactoring:

• ashr for i8 had unnecessarily complicated negative test case, so simplify it,
• shl for i128 has unnecessary complicated implementation, so simplify it,
• move sign extension to an utility function instead of a closure in smul.

Details

I couldn't figure out how llvm.sshl.sat.i8 actually works so I wrote this small program to generate test cases and then I figured out the logic based on them. I did the same trick for the unsigned version to validate my understanding too.

declare i8 @llvm.sshl.sat.i8(i8, i8) #0

define i8 @sshl_sat_i8(i8 %a, i8 %b) #0 {
  %1 = call i8 @llvm.sshl.sat.i8(i8 %a, i8 %b)
  ret i8 %1
}

attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }

#include <stdio.h>
#include <stdint.h>

extern uint8_t sshl_sat_i8(uint8_t a, uint8_t b);

void print_binary(uint8_t value) {
    printf("0b");
    for(int i = 7; i >= 0; i--){
        printf("%u", (value >> i) & 1);
    }
}

int main() {
    uint8_t values[] = {128, 255, 127, 1, 15, 0, 0xaa, 0x55};
    for(size_t v = 0; v < sizeof(values)/sizeof(values[0]); v++) {
        uint8_t lhs = values[v];
        for(int rhs = 0; rhs <= 7; rhs++) {
            uint8_t expected = sshl_sat_i8(lhs, rhs);
            printf("TestCaseTwoArgs { lhs: ");
            print_binary(lhs);
            printf(", rhs: %d, expected: ", rhs);
            print_binary(expected);
            printf(" },\n");
        }
        printf("\n");
    }

    return 0;
}

clang -c sshl_sat.c -o sshl_sat.o
clang -c sshl_sat.ll -o sshl_sat.ll.o
clang sshl_sat.o sshl_sat.ll.o -o sshl_sat

Checklist

• Code is formatted by Rustfmt or scarb fmt.
• Documentation has been updated if necessary.

[iamrecursion]

Looks good, but I have some very crucial asks for documentation.

[iamrecursion]

LGTM!