[LLVMbugs] [Bug 2234] New: llvm-gcc duplicates code for no reason?

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2234

           Summary: llvm-gcc duplicates code for no reason?
           Product: new-bugs
           Version: unspecified
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org
                CC: llvmbugs at cs.uiuc.edu


Consider:
extern int bar (); 
volatile int w; 
int bar (void) { 
if (bar()) 
return 0; 
else 
return w; 
}

-O0 with opt -std-compile-opts compiles this to very reasonable code:

...
define i32 @bar() nounwind  {
entry:
        %tmp1 = tail call i32 @bar( ) nounwind          ; <i32> [#uses=1]
        %tmp2 = icmp eq i32 %tmp1, 0            ; <i1> [#uses=1]
        br i1 %tmp2, label %bb4, label %UnifiedReturnBlock
bb4:            ; preds = %entry
        %tmp5 = volatile load i32* @w, align 4          ; <i32> [#uses=1]
        ret i32 %tmp5
UnifiedReturnBlock:             ; preds = %entry
        ret i32 0
}


When using llvm-gcc -O3 however, I get this crazy code:

define i32 @bar() nounwind  {
entry:
        %tmp17 = tail call i32 @bar( ) nounwind                 ; <i32>
[#uses=1]
        %tmp18 = icmp eq i32 %tmp17, 0          ; <i1> [#uses=1]
        br i1 %tmp18, label %bb20, label %bb29
bb20:           ; preds = %entry
        %tmp21 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp103 = icmp eq i32 %tmp21, 0              ; <i1> [#uses=1]
        br i1 %phitmp103, label %bb29, label %bb38
bb29:           ; preds = %bb20, %entry
        %tmp30 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp102 = icmp eq i32 %tmp30, 0              ; <i1> [#uses=1]
        br i1 %phitmp102, label %bb38, label %bb47
bb38:           ; preds = %bb29, %bb20
        %tmp39 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp101 = icmp eq i32 %tmp39, 0              ; <i1> [#uses=1]
        br i1 %phitmp101, label %bb47, label %bb56
bb47:           ; preds = %bb38, %bb29
        %tmp48 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp100 = icmp eq i32 %tmp48, 0              ; <i1> [#uses=1]
        br i1 %phitmp100, label %bb56, label %bb65
bb56:           ; preds = %bb47, %bb38
        %tmp57 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp99 = icmp eq i32 %tmp57, 0               ; <i1> [#uses=1]
        br i1 %phitmp99, label %bb65, label %bb74
bb65:           ; preds = %bb56, %bb47
        %tmp66 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp98 = icmp eq i32 %tmp66, 0               ; <i1> [#uses=1]
        br i1 %phitmp98, label %bb74, label %bb83
bb74:           ; preds = %bb65, %bb56
        %tmp75 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp97 = icmp eq i32 %tmp75, 0               ; <i1> [#uses=1]
        br i1 %phitmp97, label %bb83, label %bb92
bb83:           ; preds = %bb74, %bb65
        %tmp84 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        %phitmp = icmp eq i32 %tmp84, 0         ; <i1> [#uses=1]
        br i1 %phitmp, label %bb92, label %UnifiedReturnBlock
bb92:           ; preds = %bb83, %bb74
        %tmp93 = volatile load i32* @w, align 4         ; <i32> [#uses=1]
        ret i32 %tmp93
UnifiedReturnBlock:             ; preds = %bb83
        ret i32 0
}

This is a miscompilation, because it causes multiple accesses to the volatile
value, and seems pretty pointless in general.


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