[cfe-dev] Unnecessary code generated in LLVM IR

[Craig Topper via cfe-dev]

That code looks like it was compiled without any optimizations enabled. The
IR without optimizations is meant to be easy for the frontend to generate.
Even with the code split into two asserts, without optimizations enabled I
see a lot of the same things unnecessary things.

~Craig


On Thu, Jan 30, 2020 at 12:25 AM Akash Banerjee via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Please consider the following C code:
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> *#include <math.h>int main(void) {  double c = ceil(-INFINITY);
> assert(isinf(INFINITY) && signbit(c));}*
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> The following LLVM IR is generated for the above code by clang:
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> *define dso_local i32 @main() #0 {entry:  %retval = alloca i32, align 4
> %c = alloca double, align 8  store i32 0, i32* %retval, align 4  %0 = call
> double @llvm.ceil.f64(double 0xFFF0000000000000)  store double %0, double*
> %c, align 8  br i1 true, label %cond.true, label %cond.false
>   //Unnecessary conditional branchcond.true:
>          ; preds = %entry  %call = call i32 @__isinff(float
> 0x7FF0000000000000) #4  %tobool = icmp ne i32 %call, 0  br i1 %tobool,
> label %land.rhs, label %land.endcond.false:
>       ; preds = %entry**    //Unnecessary Block*
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> *  br i1 false, label %cond.true1, label %cond.false4cond.true1:
>                             ; preds = %cond.false*
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> *   //Unnecessary Block  %call2 = call i32 @__isinf(double
> 0x7FF0000000000000) #4  %tobool3 = icmp ne i32 %call2, 0  br i1 %tobool3,
> label %land.rhs, label %land.endcond.false4:
>        ; preds = %cond.false   //Unnecessary Block  %call5 = call i32
> @__isinfl(x86_fp80 0xK7FFF8000000000000000) #4  %tobool6 = icmp ne i32
> %call5, 0  br i1 %tobool6, label %land.rhs, label %land.endland.rhs:
>                                   ; preds = %cond.false4, %cond.true1,
> %cond.true  %1 = load double, double* %c, align 8  %2 = bitcast double %1
> to i64  %3 = icmp slt i64 %2, 0  br label %land.endland.end:
>                           ; preds = %land.rhs, %cond.false4, %cond.true1,
> %cond.true  %4 = phi i1 [ false, %cond.false4 ], [ false, %cond.true1 ], [
> false, %cond.true ], [ %3, %land.rhs ]  %land.ext = zext i1 %4 to i32
> %call7 = call i32 (i32, ...) bitcast (i32 (...)* @assert to i32 (i32,
> ...)*)(i32 %land.ext)  %5 = load i32, i32* %retval, align 4  ret i32 %5}*
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> This seems to happen because of the logical and operator in the assert,
> and instead, if the assert is split into two separate asserts:
> *    assert(isinf(INFINITY));*
> *    assert(signbit(c));*
> , then the IR generated is fine.
>
> Thanks,
> Akash.
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> cfe-dev at lists.llvm.org
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>
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