[LLVMbugs] [Bug 17211] New: FMA not generated with -ffp-contract=on

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=17211

            Bug ID: 17211
           Summary: FMA not generated with -ffp-contract=on
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Driver
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kkhoo at perfwizard.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

With the following code, I expected to see a "vfmadd*" instruction generated.
I'm not sure what functional differences are enabled by the fp-contract
parameter. It appears to accept "off", "on", and "fast". If I use "fast", I get
the expected codegen.


$ ./clang -v
clang version 3.4 (trunk 189776)
Target: x86_64-apple-darwin11.4.2
Thread model: posix

$ cat fma.c 
double foo(double x) {
    return x*x + x;
}

$ ./clang -S -O3 -ffp-contract=on -march=core-avx2 -o /dev/stdout fma.c
    .section    __TEXT,__text,regular,pure_instructions
    .globl    _foo
    .align    4, 0x90
_foo:                                   ## @foo
    .cfi_startproc
## BB#0:                                ## %entry
    pushq    %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    vmulsd    %xmm0, %xmm0, %xmm1
    vaddsd    %xmm0, %xmm1, %xmm0
    popq    %rbp
    ret
    .cfi_endproc

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