[llvm-bugs] [Bug 33199] New: ARM: inefficient repeated floating-point to int conversion

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33199

            Bug ID: 33199
           Summary: ARM: inefficient repeated floating-point to int
                    conversion
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: gergo.barany at inria.fr
                CC: llvm-bugs at lists.llvm.org

Created attachment 18525
  --> https://bugs.llvm.org/attachment.cgi?id=18525&action=edit
Input C file for triggering the issue

Simple test case:

$ cat tst.c
int fn1(double c, int *p) {
  int i = (int) c;
  *p = i;
  return i;
}

On the source level, the double-to-int conversion is done once, but Clang
duplicates it in the generated code:

$ clang --target=armv7a-eabihf -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard
-O3 -S tst.c -o -
[...]
fn1:
        .fnstart
@ BB#0:
        vcvt.s32.f64    s2, d0
        vcvt.s32.f64    s0, d0
        vstr    s2, [r0]
        vmov    r0, s0
        bx      lr
.Lfunc_end0:
[...]

This is for VFPv3, but it behaves identically with -mfpu=neon. A simple
duplication may not be a big deal, but the conversion is systematically
duplicated if I add more uses of the converted value:

$ cat tst.c
int fn1(double c, int *p, int *q, int *r, int *s) {
  int i = (int) c;
  *p = i;
  *q = i;
  *r = i;
  *s = i;
  return i;
}

$ clang --target=armv7a-eabihf -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard
-O3 -S tst.c -o -
[...]
        vcvt.s32.f64    s2, d0
        vstr    s2, [r0]
        vcvt.s32.f64    s2, d0
        vstr    s2, [r1]
        vcvt.s32.f64    s2, d0
        vstr    s2, [r2]
        vcvt.s32.f64    s2, d0
        vcvt.s32.f64    s0, d0
        vmov    r0, s0
        vstr    s2, [r3]
        bx      lr

In all cases, GCC only does the conversion once.

$ clang --version
clang version 5.0.0 (trunk 303270)

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